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Default type

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:

a=fmtp:101 0-11
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:

  • 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.