I have been banging my head against my wall on this the past 4 months with TAC and AT&T pointing the finger at each other. Hoping someone else has experience this and found a solution.
Outbound call flow: IP Phone > CUCM 12.5 > Cisco 3925 CUBE > AT&T IP Flex
Inbound call flow AT&T IP Flex > Cisco 3925 CUBE > CUCM 12.5 > IP Phone
We used TDS up until December, 2020 when we moved to AT&T. Blind/unattended transfers have not worked since. So far Cisco has made one change. They had us check "Send send-receive SDP in mid-call INVITE" on the SIP profile. AT&T claims to not have made any changes, although I doubt that as blind transfers are now working for some instances.
Blind transfers to all cellphones are still failing. Here is an example: Cell phone user calls into our DID. The DID user then presses the transfer softkey and dials another cell phone number to start the transfer. The original caller is connected to MOH and hears the tune. When the DID user hears ringing, he presses the transfer softkey again and from his perspective it appears to have succeeded and the call ends. However, the original caller hears only silence for about 4 seconds (the MOH stops) before disconnection. No ringback is heard. The final party (cell phone that the DID attempted the transfer to) does hear ringing. IF he were to pick up the call within a couple seconds, it would connect. However, it would never make it to his voicemail were he to just let it ring.
We still have a PRI from TDS, and if we recreate the exact same call out that PRI, it works perfectly.
The same type of transfer works if the final destination is an external POTS/landline. For example
Cell phone user calls into our DID, The DID user then presses transfer softkey and dials an external POTS line, the call goes back out the CUBE. The DID user presses transfer again after hearing ringing and the call ends from his perspective. The party that was transferred hears music on hold until transfer is pressed for the second time. He then hears ringback and the call is either answered or makes it to voicemail.
Blind transfers also work to internal numbers.
We are at a stalemate between AT&T and TAC. TAC is saying that the CUBE is expecting a 200 OK from AT&T during the transfer, it is not getting it, so the call times out and a BYE is sent from the CUBE. AT&T is saying it would only send a 200 OK if the call was answered.
I have attached a diagram of a failed blind transfer to this post.
This is a shot in the dark, but if anyone else has experienced this, I would love to know how it was resolved.