On Oct 24, 2021, at 11:37 AM, Dan Cross <crossd@gmail.com> wrote:

On Sat, Oct 23, 2021, 7:50 PM John Floren <john@jfloren.net> wrote:
I'm experimenting with my PiDP-11; I think I may have my modem set up properly to accept incoming calls, but with only one phone line I'm unable to test it. If anyone with a modem is willing to help me test, send me a message off-list and I'll give you my phone number & some login details.

I'd be less surprised by folks having modems (which seem like the kind of thing that gets tossed into a box and forgotten about for a few decades until eventually uncovered by a bout of decluttering) and more surprised by people having suitable phone lines to use them.

        - Dan C.

With a second modem, an Analog telephone adapter, and a free Google Voice account you could do it.  Might have to set the ATA to G.711 to properly pass modem signals.
Or you could set up an Asterix server and run your own private phone network.

I am surprised there doesn't seem to be an open source soft-modem that directly speaks SIP protocols, but there is a commercial one at https://www.vocal.com/voip/sip-software-modem/