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:
- a=fmtp:101 0-15
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.
Changing the DTMF type
You can change the DTMF type as follows:
- Default: RFC 2833 is used
- To enable SIP INFO in addition to RFC 2833, you can set "DTMF via SIP INFO" - on WebUI of phone → Identity → SIP to "on"
- To enable SIP INFO only, you can set "DTMF via SIP INFO" to "sip_info_only"
- To force in-band DTMF: set "DTMF via SIP INFO" to "on" or to "off" (but not to "sip_info_only"), and under phone -> Identity 1 -> RTP remove "telephone-event" from the Codec List.