There could be an issue where DTMF tones are not making it all the way to UMS.
You should turn on UMS logging to see if the tones are arriving.
Here's how to turn on the UMS logging:
cd /opt/3com/VCX/UMS/app/remoteCli/bin
./remoteCli ums
config sipc logi=true
config cbilog loga=true
exit
The capture logs will be in
/opt/3com/VCX/UMS/app/logs
They are:
ums_cbipi.log
ums_sippi1.log
Then turn off UMS logging by changing true to false in the above statements.
Depending on your version of VCX, the log files might be in a different place.
You can always do this:
cd /opt/3com/VCX/UMS/app
find . -name ums_cbipi.log