OpenVoIP on anonradio is fun. Dial into a party line extension on the air.

SDFers can get access to a SIP extension. I think it's enabled by way of the maint command from the shell. From there, you can use a softphone like linfone to communicate with other SDFers.

The SDF VoIP service is also connected to the C*NET telephone collectors gateway and you can dial around to explore that network.

I paid for a DID for a while and connected to that with a grandstream DECT base station with a few wireless phones as well as a grandstream ATA and an old western digital phone. For a long time that was the only phone service in my home.

Look at the join and tutorials pages on sdf.org for all sorts of other info. There's countless services on the fortress. Some old, some new, all in various states of maintenance. Keep digging. You'll find some cool stuff.