FAQ/How can I change settings in-band and out-of-band DTMF signalling

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Answer

The DTMF type is negotiated during the call setup:

By default, the snom phone (SIP UA) sends the following message inside the INVITE message SDP body:

a=fmtp:101 0-15

where

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 OOB the SIP OK response may contain the following in the SDP section:

a=fmtp:101 0-11

Note: If this message is absent, the snom phone assumes in-band DTMF to 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.

To enable SIP INFO, you can set "DTMF via SIP INFO" to "on".

To force in-band DTMF: set "DTMF via SIP INFO" to "off", and under phone -> Identity 1 -> RTP remove "telephone-event" from the Codec-Liste.

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