5.10.1.4. Incoming DTMF functionality 
For incoming network connection, the DTMF activation via SUFDIAL (DTMF
access code) is allowed. As soon the DTMF access code is dialed, the
called party allocates the MF circuit sender to convert pressed digits
into DTMF signals.
Alternatively, for trunks it is possible to set COT parameters AMFC
(COT IBBA may also be needed, depending on the customer requirement):
- AMFC - allows in-band DTMF functionality after the call is answered.
- IBBA - allows in-band DTMF functionality before the call is answered (while still in ring state).
IMPORTANT:
DTMF negotiation may also take place in these scenarios.