We’ve seen similar issues with some PRI providers; the AdTran doesn’t provide a long enough DTMF for the system to always pick it up, so we’d see randomly dropped/missed key presses from incoming callers. It took MONTHS of working with the provider before they added this to the AdTran config and all the problems went away:
ip rtp dtmf-relay min-duration 250
This might not be exactly your problem, but might also point you in the right direction.