Ref: draft-ietf-rtcweb-audio-08 section 3 requires support, as does <http://w3c.github.io/webrtc-pc/#peer-to-peer-dtmf>. These documents are pretty mature, and quite unlikely to change in this regard. This bug covers the SDP changes that are needed. We are filing a separate bug to cover the JS API.
Mass change P2->P3 to align with new Mozilla triage process.