Airtime does not handle phone systems natively. But it is very easy to hookup third party utilities that broadcast a stream to Airtime via Icecast or Shoutcast protocol, which is then rebroadcast by Airtime.
Some examples of utilities that Airtime can use as an input are
IDJC
Mixxx
Edcast
BUTT
As far as I'm aware none of these handle phone calls. If you find one that works, please let us know.