I'm using pfSense 2.1.5 on a PC Engines APU.1C Board. I assigned the E1820 USB UMTS stick as interface and configured PPP.

Here is my usbconfig:

$ usbconfig
ugen0.1: <ohci root="" hub="" ati=""> at usbus0, cfg=0 md=HOST spd=FULL (12Mbps) pwr=SAVE
ugen1.1: <ehci root="" hub="" ati=""> at usbus1, cfg=0 md=HOST spd=HIGH (480Mbps) pwr=SAVE
ugen2.1: <ohci root="" hub="" ati=""> at usbus2, cfg=0 md=HOST spd=FULL (12Mbps) pwr=SAVE
ugen3.1: <ehci root="" hub="" ati=""> at usbus3, cfg=0 md=HOST spd=HIGH (480Mbps) pwr=SAVE
ugen4.1: <ohci root="" hub="" ati=""> at usbus4, cfg=0 md=HOST spd=FULL (12Mbps) pwr=SAVE
ugen5.1: <ohci root="" hub="" ati=""> at usbus5, cfg=0 md=HOST spd=FULL (12Mbps) pwr=SAVE
ugen6.1: <ehci root="" hub="" ati=""> at usbus6, cfg=0 md=HOST spd=HIGH (480Mbps) pwr=SAVE
ugen6.2: <flash card="" readerwriter="" generic=""> at usbus6, cfg=0 md=HOST spd=HIGH (480Mbps) pwr=ON
ugen1.2: <huawei mobile="" huawei="" technologies=""> at usbus1, cfg=0 md=HOST spd=HIGH (480Mbps) pwr=ON</huawei></flash></ehci></ohci></ohci></ehci></ohci></ehci></ohci>

Also here my cua:

$ dir /dev/cua*;dmesg
Copyright (c) 1992-2012 The FreeBSD Project.
Copyright (c) 1979, 1980, 1983, 1986, 1988, 1989, 1991, 1992, 1993, 1994
	The Regents of the University of California. All rights reserved.
FreeBSD is a registered trademark of The FreeBSD Foundation.
FreeBSD 8.3-RELEASE-p16 #0: Mon Aug 25 08:27:41 EDT 2014
    root@pf2_1_1_i386.pfsense.org:/usr/obj.i386/usr/pfSensesrc/src/sys/pfSense_wrap.8.i386 i386
Timecounter "i8254" frequency 1193182 Hz quality 0
CPU: AMD G-T40E Processor (1000.00-MHz 686-class CPU)
  Origin = "AuthenticAMD"  Id = 0x500f20  Family = 14  Model = 2  Stepping = 0
  Features=0x178bfbff <fpu,vme,de,pse,tsc,msr,pae,mce,cx8,apic,sep,mtrr,pge,mca,cmov,pat,pse36,clflush,mmx,fxsr,sse,sse2,htt>Features2=0x802209 <sse3,mon,ssse3,cx16,popcnt>AMD Features=0x2e500800 <syscall,nx,mmx+,ffxsr,page1gb,rdtscp,lm>AMD Features2=0x35ff <lahf,cmp,svm,extapic,cr8,abm,sse4a,mas,prefetch,ibs,skinit,wdt>TSC: P-state invariant
real memory  = 2115289088 (2017 MB)
avail memory = 2051104768 (1956 MB)
ACPI APIC Table: <core  coreboot="">
FreeBSD/SMP: Multiprocessor System Detected: 2 CPUs
FreeBSD/SMP: 1 package(s) x 2 core(s)
 cpu0 (BSP): APIC ID:  0
 cpu1 (AP): APIC ID:  1
ioapic0 <version 2.1=""> irqs 0-23 on motherboard
ipw_bss: You need to read the LICENSE file in /usr/share/doc/legal/intel_ipw/.
ipw_bss: If you agree with the license, set legal.intel_ipw.license_ack=1 in /boot/loader.conf.
module_register_init: MOD_LOAD (ipw_bss_fw, 0xc0738f60, 0) error 1
ipw_ibss: You need to read the LICENSE file in /usr/share/doc/legal/intel_ipw/.
ipw_ibss: If you agree with the license, set legal.intel_ipw.license_ack=1 in /boot/loader.conf.
module_register_init: MOD_LOAD (ipw_ibss_fw, 0xc0739000, 0) error 1
ipw_monitor: You need to read the LICENSE file in /usr/share/doc/legal/intel_ipw/.
ipw_monitor: If you agree with the license, set legal.intel_ipw.license_ack=1 in /boot/loader.conf.
module_register_init: MOD_LOAD (ipw_monitor_fw, 0xc07390a0, 0) error 1
wlan: mac acl policy registered
cryptosoft0: <software crypto=""> on motherboard
padlock0: No ACE support.
acpi0: <core coreboot=""> on motherboard
acpi0: [ITHREAD]
acpi0: Power Button (fixed)
Timecounter "ACPI-fast" frequency 3579545 Hz quality 1000
acpi_timer0: <32-bit timer at 3.579545MHz> port 0x808-0x80b on acpi0
cpu0: <acpi cpu=""> on acpi0
cpu1: <acpi cpu=""> on acpi0
acpi_hpet0: <high precision="" event="" timer=""> iomem 0xfed00000-0xfed003ff on acpi0
Timecounter "HPET" frequency 14318180 Hz quality 900
pcib0: <acpi host-pci="" bridge=""> port 0xcf8-0xcff iomem 0xa0000-0xbffff on acpi0
pci0: <acpi pci="" bus=""> on pcib0
pcib1: <acpi pci-pci="" bridge=""> irq 16 at device 4.0 on pci0
pci1: <acpi pci="" bus=""> on pcib1
re0: <realtek 8111="" 8168="" b="" c="" cp="" d="" dp="" e="" f="" pcie="" gigabit="" ethernet=""> port 0x1000-0x10ff mem 0xf7900000-0xf7900fff,0xf7800000-0xf7803fff irq 16 at device 0.0 on pci1
re0: Using 1 MSI-X message
re0: ASPM disabled
re0: Chip rev. 0x2c000000
re0: MAC rev. 0x00000000
miibus0: <mii bus=""> on re0
rgephy0: <rtl8169s 8110s="" 8211b="" media="" interface=""> PHY 1 on miibus0
rgephy0:  none, 10baseT, 10baseT-FDX, 10baseT-FDX-flow, 100baseTX, 100baseTX-FDX, 100baseTX-FDX-flow, 1000baseT, 1000baseT-master, 1000baseT-FDX, 1000baseT-FDX-master, 1000baseT-FDX-flow, 1000baseT-FDX-flow-master, auto, auto-flow
re0: [ITHREAD]
pcib2: <acpi pci-pci="" bridge=""> irq 17 at device 5.0 on pci0
pci2: <acpi pci="" bus=""> on pcib2
re1: <realtek 8111="" 8168="" b="" c="" cp="" d="" dp="" e="" f="" pcie="" gigabit="" ethernet=""> port 0x2000-0x20ff mem 0xf7b00000-0xf7b00fff,0xf7a00000-0xf7a03fff irq 17 at device 0.0 on pci2
re1: Using 1 MSI-X message
re1: ASPM disabled
re1: Chip rev. 0x2c000000
re1: MAC rev. 0x00000000
miibus1: <mii bus=""> on re1
rgephy1: <rtl8169s 8110s="" 8211b="" media="" interface=""> PHY 1 on miibus1
rgephy1:  none, 10baseT, 10baseT-FDX, 10baseT-FDX-flow, 100baseTX, 100baseTX-FDX, 100baseTX-FDX-flow, 1000baseT, 1000baseT-master, 1000baseT-FDX, 1000baseT-FDX-master, 1000baseT-FDX-flow, 1000baseT-FDX-flow-master, auto, auto-flow
re1: [ITHREAD]
pcib3: <acpi pci-pci="" bridge=""> irq 18 at device 6.0 on pci0
pci3: <acpi pci="" bus=""> on pcib3
re2: <realtek 8111="" 8168="" b="" c="" cp="" d="" dp="" e="" f="" pcie="" gigabit="" ethernet=""> port 0x3000-0x30ff mem 0xf7d00000-0xf7d00fff,0xf7c00000-0xf7c03fff irq 18 at device 0.0 on pci3
re2: Using 1 MSI-X message
re2: ASPM disabled
re2: Chip rev. 0x2c000000
re2: MAC rev. 0x00000000
miibus2: <mii bus=""> on re2
rgephy2: <rtl8169s 8110s="" 8211b="" media="" interface=""> PHY 1 on miibus2
rgephy2:  none, 10baseT, 10baseT-FDX, 10baseT-FDX-flow, 100baseTX, 100baseTX-FDX, 100baseTX-FDX-flow, 1000baseT, 1000baseT-master, 1000baseT-FDX, 1000baseT-FDX-master, 1000baseT-FDX-flow, 1000baseT-FDX-flow-master, auto, auto-flow
re2: [ITHREAD]
pcib4: <acpi pci-pci="" bridge=""> irq 19 at device 7.0 on pci0
pci4: <acpi pci="" bus=""> on pcib4
ath0: <atheros 9280=""> mem 0xf7e00000-0xf7e0ffff irq 19 at device 0.0 on pci4
ath0: [ITHREAD]
[ath] enabling AN_TOP2_FIXUP
ath0: AR9280 mac 128.2 RF5133 phy 13.0
atapci0: <ati 800="" ixp700="" sata300="" controller=""> port 0x4010-0x4017,0x4020-0x4023,0x4018-0x401f,0x4024-0x4027,0x4000-0x400f mem 0xf7f08000-0xf7f083ff irq 19 at device 17.0 on pci0
atapci0: [ITHREAD]
atapci0: AHCI v1.20 controller with 6 6Gbps ports, PM supported
ata2: <ata channel=""> at channel 0 on atapci0
ata2: [ITHREAD]
ata3: <ata channel=""> at channel 1 on atapci0
ata3: [ITHREAD]
ata4: <ata channel=""> at channel 2 on atapci0
ata4: [ITHREAD]
ata5: <ata channel=""> at channel 3 on atapci0
ata5: [ITHREAD]
ata6: <ata channel=""> at channel 4 on atapci0
ata6: [ITHREAD]
ata7: <ata channel=""> at channel 5 on atapci0
ata7: [ITHREAD]
ohci0: <ohci (generic)="" usb="" controller=""> mem 0xf7f04000-0xf7f04fff irq 18 at device 18.0 on pci0
ohci0: [ITHREAD]
usbus0: <ohci (generic)="" usb="" controller=""> on ohci0
ehci0: <ehci (generic)="" usb="" 2.0="" controller=""> mem 0xf7f08400-0xf7f084ff irq 17 at device 18.2 on pci0
ehci0: [ITHREAD]
usbus1: EHCI version 1.0
usbus1: <ehci (generic)="" usb="" 2.0="" controller=""> on ehci0
ohci1: <ohci (generic)="" usb="" controller=""> mem 0xf7f05000-0xf7f05fff irq 18 at device 19.0 on pci0
ohci1: [ITHREAD]
usbus2: <ohci (generic)="" usb="" controller=""> on ohci1
ehci1: <ehci (generic)="" usb="" 2.0="" controller=""> mem 0xf7f08500-0xf7f085ff irq 17 at device 19.2 on pci0
ehci1: [ITHREAD]
usbus3: EHCI version 1.0
usbus3: <ehci (generic)="" usb="" 2.0="" controller=""> on ehci1
pci0: <serial bus,="" smbus=""> at device 20.0 (no driver attached)
isab0: <pci-isa bridge=""> at device 20.3 on pci0
isa0: <isa bus=""> on isab0
pcib5: <acpi pci-pci="" bridge=""> at device 20.4 on pci0
pci5: <acpi pci="" bus=""> on pcib5
ohci2: <ohci (generic)="" usb="" controller=""> mem 0xf7f06000-0xf7f06fff irq 18 at device 20.5 on pci0
ohci2: [ITHREAD]
usbus4: <ohci (generic)="" usb="" controller=""> on ohci2
pcib6: <acpi pci-pci="" bridge=""> at device 21.0 on pci0
pci6: <acpi pci="" bus=""> on pcib6
ohci3: <ohci (generic)="" usb="" controller=""> mem 0xf7f07000-0xf7f07fff at device 22.0 on pci0
ohci3: [ITHREAD]
usbus5: <ohci (generic)="" usb="" controller=""> on ohci3
ehci2: <ehci (generic)="" usb="" 2.0="" controller=""> mem 0xf7f08600-0xf7f086ff at device 22.2 on pci0
ehci2: [ITHREAD]
usbus6: EHCI version 1.0
usbus6: <ehci (generic)="" usb="" 2.0="" controller=""> on ehci2
acpi_button0: <power button=""> on acpi0
atrtc0: <at realtime="" clock=""> port 0x70-0x71 irq 8 on acpi0
orm0: <isa option="" rom=""> at iomem 0xee800-0xeffff pnpid ORM0000 on isa0
ata0: <ata channel=""> at port 0x1f0-0x1f7,0x3f6 irq 14 on isa0
ata0: [ITHREAD]
ata1: <ata channel=""> at port 0x170-0x177,0x376 irq 15 on isa0
ata1: [ITHREAD]
ppc0: <parallel port=""> at port 0x378-0x37f irq 7 on isa0
ppc0: Generic chipset (EPP/NIBBLE) in COMPATIBLE mode
ppc0: [ITHREAD]
ppbus0: <parallel port="" bus=""> on ppc0
ppi0: <parallel i="" o=""> on ppbus0
uart0: <16550 or compatible> at port 0x3f8-0x3ff irq 4 flags 0x10 on isa0
uart0: [FILTER]
uart0: console (9600,n,8,1)
uart1: <16550 or compatible> at port 0x2f8-0x2ff irq 3 on isa0
uart1: [FILTER]
acpi_throttle0: <acpi cpu="" throttling=""> on cpu0
acpi_throttle1: <acpi cpu="" throttling=""> on cpu1
acpi_throttle1: failed to attach P_CNT
device_attach: acpi_throttle1 attach returned 6
RTC BIOS diagnostic error ff <clock_battery,rom_cksum,config_unit,memory_size,fixed_disk,invalid_time>Timecounters tick every 10.000 msec
IPsec: Initialized Security Association Processing.
usbus0: 12Mbps Full Speed USB v1.0
usbus1: 480Mbps High Speed USB v2.0
ugen0.1: <ati> at usbus0
uhub0: <ati 1="" 9="" ohci="" root="" hub,="" class="" 0,="" rev="" 1.00="" 1.00,="" addr=""> on usbus0
ugen1.1: <ati> at usbus1
uhub1: <ati 1="" 9="" ehci="" root="" hub,="" class="" 0,="" rev="" 2.00="" 1.00,="" addr=""> on usbus1
usbus2: 12Mbps Full Speed USB v1.0
usbus3: 480Mbps High Speed USB v2.0
ugen2.1: <ati> at usbus2
uhub2: <ati 1="" 9="" ohci="" root="" hub,="" class="" 0,="" rev="" 1.00="" 1.00,="" addr=""> on usbus2
ugen3.1: <ati> at usbus3
uhub3: <ati 1="" 9="" ehci="" root="" hub,="" class="" 0,="" rev="" 2.00="" 1.00,="" addr=""> on usbus3
usbus4: 12Mbps Full Speed USB v1.0
usbus5: 12Mbps Full Speed USB v1.0
ugen4.1: <ati> at usbus4
uhub4: <ati 1="" 9="" ohci="" root="" hub,="" class="" 0,="" rev="" 1.00="" 1.00,="" addr=""> on usbus4
ugen5.1: <ati> at usbus5
uhub5: <ati 1="" 9="" ohci="" root="" hub,="" class="" 0,="" rev="" 1.00="" 1.00,="" addr=""> on usbus5
usbus6: 480Mbps High Speed USB v2.0
uhub0: 5 ports with 5 removable, self powered
ugen6.1: <ati> at usbus6
uhub6: <ati 1="" 9="" ehci="" root="" hub,="" class="" 0,="" rev="" 2.00="" 1.00,="" addr=""> on usbus6
uhub4: 2 ports with 2 removable, self powered
uhub2: 5 ports with 5 removable, self powered
uhub5: 4 ports with 4 removable, self powered
uhub1: 5 ports with 5 removable, self powered
uhub3: 5 ports with 5 removable, self powered
uhub6: 4 ports with 4 removable, self powered
ugen6.2: <generic> at usbus6
umass0: <generic 0="" 2="" flash="" card="" readerwriter,="" class="" 0,="" rev="" 2.01="" 1.00,="" addr=""> on usbus6
ugen1.2: <huawei technologies=""> at usbus1
ugen1.2: <huawei technologies=""> at usbus1 (disconnected)
SMP: AP CPU #1 Launched!
da0 at umass-sim0 bus 0 scbus0 target 0 lun 0
da0: <multiple card ="" reader="" 1.00=""> Removable Direct Access SCSI-4 device 
da0: 40.000MB/s transfers
da0: 7580MB (15523840 512 byte sectors: 255H 63S/T 966C)
Trying to mount root from ufs:/dev/da0s1a
WARNING: / was not properly dismounted
ZFS NOTICE: Prefetch is disabled by default on i386 -- to enable,
            add "vfs.zfs.prefetch_disable=0" to /boot/loader.conf.
ZFS WARNING: Recommended minimum kmem_size is 512MB; expect unstable behavior.
             Consider tuning vm.kmem_size and vm.kmem_size_max
             in /boot/loader.conf.
ZFS filesystem version 5
ZFS storage pool version 28
ugen1.2: <huawei technologies=""> at usbus1
u3g0: <huawei 0="" 2="" technologies="" huawei="" mobile,="" class="" 0,="" rev="" 2.00="" 0.00,="" addr=""> on usbus1
u3g0: Found 4 ports.
umass1: <huawei 0="" 2="" technologies="" huawei="" mobile,="" class="" 0,="" rev="" 2.00="" 0.00,="" addr=""> on usbus1
umass2: <huawei 0="" 2="" technologies="" huawei="" mobile,="" class="" 0,="" rev="" 2.00="" 0.00,="" addr=""> on usbus1
(probe0:umass-sim1:1:0:0): TEST UNIT READY. CDB: 0 0 0 0 0 0 
(probe0:umass-sim1:1:0:0): CAM status: SCSI Status Error
(probe0:umass-sim1:1:0:0): SCSI status: Check Condition
(probe0:umass-sim1:1:0:0): SCSI sense: NOT READY asc:3a,0 (Medium not present)
(probe1:umass-sim2:2:0:0): TEST UNIT READY. CDB: 0 0 0 0 0 0 
(probe1:umass-sim2:2:0:0): CAM status: SCSI Status Error
(probe1:umass-sim2:2:0:0): SCSI status: Check Condition
(probe1:umass-sim2:2:0:0): SCSI sense: NOT READY asc:3a,0 (Medium not present)
cd0 at umass-sim1 bus 1 scbus1 target 0 lun 0
cd0: <huawei mass="" storage="" 2.31=""> Removable CD-ROM SCSI-2 device 
cd0: 40.000MB/s transfers
cd0: Attempt to query device size failed: NOT READY, Medium not present
da1 at umass-sim2 bus 2 scbus2 target 0 lun 0
da1: <huawei tf="" card="" storage=""> Removable Direct Access SCSI-2 device 
da1: 40.000MB/s transfers
da1: Attempt to query device size failed: NOT READY, Medium not present
wlan0: changing name to 'ath0_wlan0'
re1: link state changed to DOWN
re1: link state changed to UP
re2: link state changed to DOWN
ng0: changing name to 'ppp0'
DUMMYNET 0 with IPv6 initialized (100409)
load_dn_sched dn_sched FIFO loaded
load_dn_sched dn_sched QFQ loaded
load_dn_sched dn_sched RR loaded
load_dn_sched dn_sched WF2Q+ loaded
load_dn_sched dn_sched PRIO loaded
re2: link state changed to UP
pflog0: promiscuous mode enabled
ath0: stuck beacon; resetting (bmiss count 6)
ath0: stuck beacon; resetting (bmiss count 0)
ipfw2 (+ipv6) initialized, divert loadable, nat loadable, rule-based forwarding enabled, default to accept, logging disabled
ng0: changing name to 'ppp0'
ng0: changing name to 'ppp0'
ng0: changing name to 'ppp0'
ng0: changing name to 'ppp0'
ng0: changing name to 'ppp0'
re0: link state changed to DOWN
ng0: changing name to 'ppp0'
ng0: changing name to 'ppp0'
ng0: changing name to 'ppp0'
ng0: changing name to 'ppp0'
ng0: changing name to 'ppp0'
ng0: changing name to 'ppp0'</huawei></huawei></huawei></huawei></huawei></huawei></multiple></huawei></huawei></generic></generic></ati></ati></ati></ati></ati></ati></ati></ati></ati></ati></ati></ati></ati></ati></clock_battery,rom_cksum,config_unit,memory_size,fixed_disk,invalid_time></acpi></acpi></parallel></parallel></parallel></ata></ata></isa></at></power></ehci></ehci></ohci></ohci></acpi></acpi></ohci></ohci></acpi></acpi></isa></pci-isa></serial></ehci></ehci></ohci></ohci></ehci></ehci></ohci></ohci></ata></ata></ata></ata></ata></ata></ati></atheros></acpi></acpi></rtl8169s></mii></realtek></acpi></acpi></rtl8169s></mii></realtek></acpi></acpi></rtl8169s></mii></realtek></acpi></acpi></acpi></acpi></high></acpi></acpi></core></software></version></core ></lahf,cmp,svm,extapic,cr8,abm,sse4a,mas,prefetch,ibs,skinit,wdt></syscall,nx,mmx+,ffxsr,page1gb,rdtscp,lm></sse3,mon,ssse3,cx16,popcnt></fpu,vme,de,pse,tsc,msr,pae,mce,cx8,apic,sep,mtrr,pge,mca,cmov,pat,pse36,clflush,mmx,fxsr,sse,sse2,htt>

On Status Interfaces I see:

UMTS interface (ppp0)
Status	up
PPP	 
Uptime	00:00:0
Cell Signal (RSSI)	rssi:15 level:-83dBm percent:48%
Cell Mode	None, No Service Mode
Cell SIM State	Valid SIM State
Cell Service	Valid Service
MAC address	00:00:00:00:00:00
IPv6 Link Local	fe80::20d:b9ff:fe33:910%ppp0	 
In/out packets	0/0 (0 bytes/0 bytes)
In/out packets (pass)	0/0 (0 bytes/0 bytes)
In/out packets (block)	0/9 (0 bytes/660 bytes)
In/out errors	0/0
Collisions	0

System logs PPP shows:

Nov 26 23:23:04	ppp: [opt3_link0] Link: reconnection attempt 7 in 1 seconds
Nov 26 23:23:04	ppp: [opt3_link0] LCP: Down event
Nov 26 23:23:04	ppp: [opt3_link0] Link: DOWN event
Nov 26 23:23:04	ppp: [opt3_link0] MODEM: Fail to open serial port /dev/cuau0 on speed 115200
Nov 26 23:23:04	ppp: [opt3_link0] uu_lock(cuau0): read error: File exists
Nov 26 23:23:02	ppp: [opt3_link0] Link: reconnection attempt 6
Nov 26 23:23:01	ppp: [opt3_link0] Link: reconnection attempt 6 in 1 seconds
Nov 26 23:23:01	ppp: [opt3_link0] LCP: Down event
Nov 26 23:23:01	ppp: [opt3_link0] Link: DOWN event
Nov 26 23:23:01	ppp: [opt3_link0] MODEM: Fail to open serial port /dev/cuau0 on speed 115200
Nov 26 23:23:01	ppp: [opt3_link0] uu_lock(cuau0): read error: File exists
Nov 26 23:23:00	ppp: [opt3_link0] Link: reconnection attempt 5
Nov 26 23:22:58	ppp: [opt3_link0] Link: reconnection attempt 5 in 2 seconds
Nov 26 23:22:58	ppp: [opt3_link0] LCP: Down event
Nov 26 23:22:58	ppp: [opt3_link0] Link: DOWN event

What do you guys think is the problem?