Remember that the ASA is "SIP Aware" , and it's default enabled under services.
Not that it should do something to a conn limit.
Also you have port 5061 in your ASA rules , and refer to 5060 in the pfsense examples.
Someone here mentioned using siproxd , on pfsense
http://siproxd.sourceforge.net/
SIP and NAT requires some thoughts , or a sip aware proxy to "modify" "the inside ip addr" , to the "outside addr" , in the "payload"
All of that stuff is being done by default , in the ASA.
/Bingo