I have 7 Polycom IP650's at one location behind a pfSense box. We're not using Asterisk (using OnSip/Junction Networks) but I had the same problem. I tried siproxd and couldn't get it to work. After reading every post here concerning SIP (at least 3 times) I finally started with a fresh install and enabled Advanced Outbound NAT, made a rule for the VoIP VLAN out (separate interface from LAN), and rebooted. Every phone came up and has been working great since. I've never used Asterisk (been wanting to check it out in my "spare" time), but OnSip runs everything by RFC standards, no extra bells and whistles unless it's a standard. I would "assume" that Asterisk works the same way.
Of course, the great thing about standards is there are so many to choose from . . . ;)