Comment 5 for bug 1187453

I'm not sure if this is directly helpful, but:

Sailfish OS and the Jolla phone use the tone generator daemon [1] for local playback of DTMF sounds. These D-Bus calls start and stop playing a DTMF tone that is equivalent to pressing key '5' during a voice call:
  dbus-send --system --type=method_call --dest=com.Nokia.Telephony.Tones /com/Nokia/Telephony/Tones com.Nokia.Telephony.Tones.StartEventTone uint32:5 int32:0 uint32:1000
  dbus-send --system --type=method_call --dest=com.Nokia.Telephony.Tones /com/Nokia/Telephony/Tones com.Nokia.Telephony.Tones.StopTone

tone-generator has its origin in Nokia Maemo / MeeGo, and is LGPL-licensed. From a quick glance, tone-generator seems to depend only on PulseAudio, GLib/GObject and D-Bus. It should be possible to use it (or parts of it) on Ubuntu.

For reference, in Sailfish OS the tone generation daemon is used by 'voicecall manager' ([2] , roughly equivalent to Ubuntu's telephony-service) for DTMF sounds, and by telepathy-ring ([3], roughly equivalent to Ubuntu's telepathy-ofono) for call event sounds and, in some circumstances, for alerting sounds.

[1] https://github.com/nemomobile/tone-generator
[2] https://github.com/nemomobile/voicecall/blob/master/plugins/tonegend/src/tonegeneratorplugin.cpp
[3] https://github.com/nemomobile/telepathy-ring/blob/master/modem/tones.c