The DTMF type is negotiated during the call setup. By default, the Snom phones use RFC 2833 out-of-band DTMF type. To do this, it sends the following message inside the INVITE message SDP body:
fmtp is the OOB (out of band RFC 2833) offer
101 example payload ID
0-15 defines the range of keys: 0-11 (12) normal keyboard + 4 letters "abcd"
If the receiver accepts out-of-band, the SIP 200 OK response may contain the following in the SDP section:
Depending on the abilities of the two parties, "out-of-band (OOB)" is certainly preferable and "in-band" should be used as fallback only.
You can change the DTMF type as follows: