Transferred call drops after about 30 sec

I recently setup a new system with a FreePBX distro (v1.812.210.57 i386) installed. I am experiencing an issue where an incoming call on a dahdi channel is transferred via an IAX trunk to an office in a different location. The destination location has an old system with PIAF 1.2 (Asterisk v1.4.19.2) installed. The call is is picked up and dropped after about 30 seconds. This problem has not happened too many times as of yet, but I would like to rule out any issues with the new system. I know that the issue could relate to the Internet provider or if the inbound caller has poor cell phone reception, but assuming that those are non-issues, what could be wrong with the system? Does anyone have any ideas?

Presuming you have correctly set up “calltokens”, (and as the call is answered then I assume you have). Then the likely culprit is your firewall/router, at either or both ends.

IAX2 is a simple one port protocol normally on udp/4569, ensure that these sessions are not “messed with” by your routers . . . .

That port on either end is open on the firewall. Any other ideas?

No, maybe go to tcpdump