Can you post your MGCP config and a 'show ver'? There are a couple of MGCP related DTMF bugs that may be of concern. When you say it 'fails', is it not recognizing anything, or is it recognizing the wrong thing. I know there is a bug where DTMF bleeds through in-band, and will cause duplicate DTMF.
Post the information requested, and we can see if you are susceptible to any of these caveats.
These are the paths to get to each CCX logs through CLI. They may be helpful if you are having issues accessing RTMT or downloading logs through it.
If you want to download them you have to prefix "file get " and you can add one of the options (re...