hardwire modem to modem

I’ve set up a RAS server, and I want to do some testing, but I don’t have two phone lines available, so I’d like to connect two modems directly using a regular UTP cable. [I don’t want to use any other method to connect them, since I want to test the modem as well.] It seems to me I used to have a way to trick the modems into starting a conversation, but I can remember what it is….