Trunk to Trunk calls dropping

Using FreePBX 2.9. Dahdi PRI trunking.
Have a ring group setup to go to another ring group if no answer. This ring group has a single member which is an outside call.
This works, but about 15 seconds into the call, it drops.
Have tried using a miscellaneous destination instead, same result. Watching the call on CLI shows call drop, cause 16. ISDN pri code for a basic hangup.

Have tried different outside numbers with the same result. Any ideas?

Was able to get this to work, but strange.

Created a IVR with no announcement, 2 second time out, default destination of the miscellaneous destination.

Ring group now overflows to IVR which times out and sends calls to miscellaneous destination telephone number.

This works, calls are not dropped. Strange.