So, the problem: Moving from Skype (closed source) to Mumble for meetings, while still having the possibility to conference with the occasional user of a POTS (plain old telephone system)
Skype lets you call POTS. Skype has a API that, among other things, allow you to send a copy of a call to a file or TCP port, and receive audio from same. The same API can also make call, e.g to POTS.
Pacat/parec allows you to use files as sources and sinks for PulseAudio. (which again can be used by Mumble)
Scripts must be written, tests must be done!