As per your outline of the call case I’d recommend you to reach out to your mobile service provider to have them look at this as what happens on the cell phone network is out of control for your end.
Only option that I see that it could be something on your end is if you use a SIP service for your PSTN connectivity when calling from your internal phone towards the mobile phone and when the call is sent to the voice mail service of the cellular network it negotiates another codec that you don’t have in the codec list towards the ITSP. This should be seen if you run these debugs.
debug voip ccapi inout
debug ccsip media
debug ccsip message
Especially pay attention to the media negotiation part.
