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:
Depending on the abilities of the two parties, "out-of-band (OOB)" is certainly preferable and "in-band" should be used as fallback only.
Changing the DTMF type
You can change the DTMF type as follows: