Hello and thanks for posting to the Community.
We have a couple ringback override options on the SIP trunk that might be helpful and worth a try. You will need to test out regular calls and the specific scenario to be certain. Here is what you will find on the SIP trunk:
TA908e(config-T01)#ringback override
180 - Override a 180 and send 183 with local ringback
183 - Override a 183 and send 183 with local ringback
This means if the ADTRAN is ready to send a 180, usually because we receive an Alerting on the PR side, we will send a 183 and generate ringback locally in rtp. This should help with cases where maybe the PBX is supposed to be providing ringback inband, but it does not for some reason. The ringback audio is only played out the SIP-side. So PRI to SIP, the PRI side does not get ringback from these commands.
Regards,
Geo