Check to make sure that the cable ends are fully seated in the modem and in the wall socket. If you have more than one phone extension, you may want to try your modem in a different phone socket. Call Pacific Bell and ask them to run some diagnostics on your phone line to test for noise. They typically will give disclaimers about 'in house wiring', etc, but if your are persistent, they may agree test your line for noise. Line noise may also cause 56k modems to connect at speeds far below their capability.
Some modems have a tendency to disconnect even with only a small amount of line noise.
Check the manufacturer's website or technical support for driver updates or other possible solutions. Winmodems tend to have far more problems establishing and maintaining a connection than other tpyes of modems. If you have a Winmodem, be sure you are using the latest drivers. Winmodems implement most of their functionality in software so they are cheaper, but less reliable. We may be able to help.