Using modems without phone lines

Modems work with T1 lines? Indirectly, sure, but if you can say that, you can say that about almost anything. Modems also work with long-haul microwaves and undersea fiber optics and satellite TWTAs too by that logic.

Try this instead: two modems with a length of flat satin cable plugged between them. ATA on one, ATX1D on the other (X1 to hopefully defeat dialtone detection). It worked with a generic 2400 bps modem and a Commodore 1670 perhaps 25 years ago. It should still work now.

Old pre-PC data systems which had COM (RS-232) ports could be directly connected to (dumb or smart) terminals by a simple RS-232 cable.

To remotely communicate, each device would need to add a modem to interface across regular ordinary phone lines. One modem would dial, the other would answer.

To use a "leased line" having no dial tone, then the dial and answer were replaced by other modem AT commands, or DIP switch settings to change default modem behavior.

Using a PC in place of a terminal was a drop-in replacement with a terminal emulator program like Hyperterminal. By the late '90's most PC's had an internal modem occupying a COM port# in addition to a regular serial connector identified by a different COM port# than the internal modem.

From Hyperterminal you could choose which COM port to communicate with, your serial port could be a local device, and modem was expected to be remote.

When the lesser laptops arrived having built-in modems but lacking serial ports any more, then you could no longer directly connect to a local RS-232 device. Serial to USB adapters, like USB itself, was not well developed, and drivers were not available for all operating systems.

A good solution for me was to use an early '90's external fax modem attached to the local serial device, even though it was going to be a local connection.

Then connect the phone connector on this modem to the laptop's phone connector locally using a simulated leased line.

IIRC the best simulated leased line I came up with had a couple 9V batteries in series to provide activity voltage, and the DC was blocked from the other end by a small capacitor which still allowed the signal frequencies to pass in both directions.

Analog lines will still support 56Kbaud, but many phone company lines are not high enough quality, so modern modems down-negotiate automatically.

For SIP, I wonder if he tried T.38?