asterisk users - Jan 2006 - DTMF not recognized on overseas call from cellphone

We have PSTN lines connected to FXO lines of a TDM400B.  I just got a 
complaint that overseas callers who are using cellphones sometimes find 
that DTMF digits aren't working - they press digits and the menu goes on 
as if they hadn't pressed anything.  Since it sometimes works, and other 
IVRs work over the same cellphones, it's not that the cellphone isn't 
sending the digits.  I asked if they had similar problems from landlines 
overseas and they did not.

Any ideas?  If I play around with rxgain could I overload something for 
callers who don't currently have any problems?