The SIP carrier requires that the packets come from us with port 5060 (UDP) as the source. Right now NAT is assigning various source ports that change with every re-registration. We are also sending registration requests way too often. I added the following to the 3120 config:
ip firewall nat-preserve-source-port record-source-address
Thinking that would preserve the source port but that doesn't seem to be working as when I show the policy sessions it shows 5060 source (internal IP from 7100), 5060 destination (ip of SIP carrier) but the NAT port is 1027 and 1206 at the moment.