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:
Note: If this message is absent, RFC 2833 is disabled and the Snom phone assumes that in-band DTMF must be used.
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:
Dedault: RFC 2833 is used
To enable SIP INFO additionally to RFC 2833, you can set "DTMF via SIP INFO" to "on"
To enable SIP INFO only, you can set "DTMF via SIP INFO" to "sip_info_only"