If you have two models connected to two computers. You might be able to test your setup by connecting a null modem phone cable between the phone line ports of both modem. 
Basically crossing transmit pairs from one side to the receive pairs of the other modem and vice-versa.  Not sure if it will work without dial tone from the phone line present. But there might setting in your modems configuration that would activate what is needed for such a modem to.modem direct link.

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.