There are a few bugs that can get you on DTMF:
CSCso87127
CSCsv04751
The first is on gateways before 12.4(15)T8
The second is on phone firmware before 8.4.2
Otherwise, you can adjust your audio levels on the PRI:
To make it 3 dB softer outbound:
voice-port 0/0/0:23
output attenuation 3
You can do this without shutting down service, and the command should take immediately.
hth,
nick