ATTO method is good: when exactly the Abort is sent? There are plenty of DTAP msg after the CM Service Req (Ciphering, Call Setup, Identity, …)
Have you tried deactivating the ciphering for a while? Which cipher are you using? A5/3 perhaps?
In the cipher mode cmd, the MSC sends a list of possible codecs, and it’s up to the BTS to choose one (depends on BTS capbilities and MS capabilities, received in a previous radio msg)