Unlike most other IP phones, the 7936 doesn't have native RTP-NTE support. Hence, when a call comes in from the SIP trunk and negotiates RTP-NTE, CM needs to invoke an MTP in order to translate the RTP-NTE from the media stream to SCCP signaling to the IP phone (and vise-versa).
If you have an RFC2833 capable MTP for the codec being used on the MRGL for the SIP trunk, DTMF should work. If you are invoking an MTP and you still see issues, get:
Are you getting this error “Installer User Interface Mode Not Supported. The installer cannot run in this UI mode. To specify the interface mode, use the -i command-line option, followed by the UI mode identifier. The value UI mode identifiers...
The below trick might come handy when you have to add a new node to a cluster but you don't have or is unsure of the security password for the publisher. This procedure has been around for ages.
1) Login into the CLI of the Publisher.