Setup → Identities → SIP Settings (Section): DTMF via SIP INFO
<user_dtmf_info idx="INDEX" perm="PERMISSIONFLAG">VALIDVALUE</user_dtmf_info>
Some IVR systems may need DTMF events signalled via SIP INFO messages which can be enabled here. As of firmware version 7.1.33 there are three options for providing DTMF codes.
If you want to force the use of in-band DTMF, you must remove the value telephone-event from the <codec_priority_list> of the according SIP identity. See also FAQ "What kind of DTMF types Snom phones support and how to change them" for more information.