Most of modem manuals are "content free", that is tell you how to install Windows drivers and how to plug the power supply into wall socket. On the other hand, great most of modems supports a Hayes (AT) command set, just google for it or see http://www.modemhelp.org/sets.html
Alternatively, write an extra software UART for your uC, plug it between PC and the modem and use it to capture the handshake between PC and the modem.