You're on the net - then you're not. Your modem has disconnected from the line - again!?!!
So, what caused the disconnect? Do you have a bad phone line? A noisy line? Possibly - but most times, the problem isn't in your phone line. Check your phone wiring.
When your modem initially connects (handshakes), it attempts to analyze the line conditions and select the highest reliable transmission rate. If the modem can't maintain the initial rate, it will at best, fallback to a lower rate (rate re-negotiation), or, retrain, or disconnect. Limiting your connect speed to a lower rate initially solves this type of problem.
The ability to deal with the line conditions and server modems you call is a function of the modem driver (controllerless and soft modems) or firmware (hardware-controller modems). Updating the modem firmware or driver may help prevent disconnections.
If your modem continues to disconnect, you should try disabling 56k protocols - limiting your modem to a V.34 (33.6kbps max) connect. (If you do not have a 56k-compatible line, disabling 56k is recommended anyway since it usually results in a shorter handshake and may result in a higher-rate and more reliable connection.)
Your Modem's diagnostics information may yield clues to cause of disconnection as well as the quality of your connections. In some cases, the Modem Log will indicate which end initiated a disconnection.
And, remember, there are times when the modem will disconnect even though you didn't tell it to: some ISPs have time and/or inactivity limits on their connections. Various software and connectoid settings on your computer may have options and settings that cause your connection to be disconnected.