Years ago, I managed to get two modems to establish a connection without an intervening telephone network by connecting a phone line between them. I turned off dial tone detection by issuing an ATH0 command to the answering modem. I then issued an ATA command. On the other modem, I issued an ATD command and the two modems negotiated a connection.

I was able to transfer files using the ZModem feature of the telnet program I was using at the time over that connection though it was significantly slower than just using a null modem cable.