Enable local playing of call error tones (but not alert tone, DTMF)

telepathy-ring uses tonegend (https://github.com/nemomobile/tone-generator) to play "busy" and "radio channel lost" tones when a call disconnects. This patch enables some old "dead code", but also disables cases where we do not want local tone generation: MO call alerting (using in-band audio from the network) and in-call DTMF (local tones generated by voicecall UI).