asterisk users - Jan 2006 - DTMF not working on overseas cellphone calls

I thought I sent this earlier this week, but I didn't see it.  If I 
missed it, I apologize for the resend.

We are running asterisk 1.2.2 with a TDM04B connected to PSTN lines.  On 
incoming calls from cellphones located overseas, DTMF is not recognized 
- we have many single-digit choices in our menu so the problem isn't 
that some digits aren't working, it's not listening at all.  Works fine 
from domestic landlines and cellphones and from overseas landlines.

I know the cellphones don't have a problem with DTMF, they work with 
other IVRs.  I've placed overseas calls (I'm currently in a different 
country from the asterisk machine) from both landlines and cellphones, 
and can't hear a difference in quality.

Could playing with rxgain help?  Is there any chance that I could cause 
the calls that don't have a problem to either be too loud or get 
distorted due to clipping?