Ignore this if OOL drops at the same time (you should see Online light go out/flash if it does). The modem log (available at »192.168.100.1) may have some info:
Does your phone plug into a phone jack in the wall or right into the modem? Plug your phone directly into the modem to see if it continues to drop. If OOL doesn't drop, then it's most frequently a wiring or phone issue, so you can try a few steps without having to call in (and have them tell you the same thing).