It is possible, but not very simply. You need to be able to force a manual connect, which is simple from the originator, but non-trivial from the terminating computer. If you can issue an ATD on one end, and an ATA on the other end, then they should be able to negotiate a connection.
I don't know how fast the connection would be (I used to do this back in the 2400 baud days) because of the complex constellations that we generate in high-speed modems. You may be better off if you can find a silent battery or quiet termination number, and connect both modems to the same phone line (this gives you a wet loop, rather than a dry loop, which may increase your odds of getting a higher speed connection). Understand that you will never connect faster than 33.6K, because both modems are analog attached to the network.
You are really far better off if you carry a null modem or null ethernet cable.
pansophic