If the problem happens with all calls across that gateway, you should probably review the config. Here's someplace to start...
www.cisco.com/en/US/products/sw/voicesw/ps2237/products_tech_note09186a0080094607.shtml
If only a few specific users experience the problem, and dtmf through that gateway works fine for everyone else, then the issue may be tied to their particular cell phone provider. Some cell phones all users to toggle between sending short or long dtmfs. Might be worth playing with that.
Also, you can use Unity's Port Status Monitor (found in the Tools Depot) to monitor digits received on incoming calls. If the digits dialed on the phone still aren't showing up at Unity, you might want to look at debugs from the gateway or some network captures to try to determine where they're getting dropped.