Hello,
I have the misfortune of forced to use a telco named Bezeq - the biggest telco in Israel.
The SIP trunk they provide is very problematic.
The one that I cannot overcome is that sometimes doing the call, it switches DTMF between Inband and rfc2833, and sometimes the whole DTMF sending is either inband or rfc2833 doing the entire call.
I cannot make them to be more stable in this matter (I have tried talking with them), and I cannot replace them (I have tried to do so as well).