The problem was with my Twilio SIP Configuration, not Ignition. I needed to ENABLE ‘Media Settings - Symmetric RTP’ in Twilio.
There is a setting in Twilio SIP → General called ‘Media Settings - Symmetric RTP’ that re-routes the RTP based on where the remote RTP is coming from (the incoming ignition connection) instead of using the destination negotiated in the SDP session descrip protocol.
For whatever reason, there is a difference between those two RTP destinations, so DTMF keys (inside of the RTP media stream) were not being returned to ignition. Now they are, and the voice alarm call menus work great. Image of setting in twilio web below:
