Fax and data modems built around Skyworks's ISOModem chips.
Current functional version is 4.1 (see 3.0 below):
I've also designed and printed a case to go with it. The STL files can be found under /case. Or on OnShape.
Version 3.0:
If you don't have access to a working landline or a proper telephone line simulator two modems can be connected back to back directly.
There are this and this pages, but I have found this circuit to work for me:
The voltage and resistance can be tweaked to provide ~20mA of current with the modem/phone off-hook.
If you'd like the schematic or just have a PCB made for the line simulator, check out the LineSim project included in this repo.
Once hooked up, you want to disable the dialtone detection with the command ATX3. Then you can just dial with ATDT on one modem and answer with ATA on the other.
See AN93 section 5.4 for details on all the AT commands supported.