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

Answer

This cannot be changed manually or forced by the phone but through the PBX only. The DTMF type is negotiated during the call setup:

The snom phone (SIP UA) sends the following message inside the INVITE message SDP body:

a=fmtp:101 0-16

where

fmtp is the OOB offer
101 example payload ID
0-16 defines the range of keys: 0-11 (12) normal keyboard + 5 letters "abcde"

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.