We had this same issue a while back, the MCD was upgraded to a newer version, and someone somewhere changed the Default Dynamic Payload for DTMF setting from 101 to 98. Mitel recommended setting was 101. Our SIP trunk provider looked into the issue but nothing they were able to do. This fixed the DTMF tone both ways, if someone called our IVR they could not navigate our IVR, if we called another companies IVR, we could not navigate theirs. The tone was literally to low.