Usually, disconnects are caused by something having to do with your phone line. If you have call waiting and a person tries to call you, your modem will get confused when it hears the beeping signal and hang up. If someone in your household picks up the phone on the same line your computer is on, it may disconnect. Poor line quality can cause you to be disconnected. Any pops, clicks, or static can confuse your modem enough to cause it to disconnect. These things can be caused by the phone company working on lines in your area, or by bad weather. Besides, it's always a good idea to unplug your modem when a storm is coming. Also, old house wiring and old phones plugged into the same line as your modem can cause unwanted noise in your phone line.
To disable call waiting, we can add the command from your phone company to disable call waiting, which is usually *70. Also, some phone companies require you to also purchase the option to disable your call waiting in addition to your regular call waiting fee. If you cannot disable your call waiting or have any questions about your call waiting service, contact your local phone provider. To add *70 to your dial string, go to My Computer, then Dial Up Networking. Right click on your connection and go to properties. Add *70,, to the beginning of the phone number there. The two comma's are necessary because they instruct the modem to wait a second for the next dial tone before dialing. Click OK, then close Dial Up Networking and My Computer. This will disable call waiting for the duration of your call, you do not need to re-enable it later. If you can not connect after you make this change, check to be sure you entered the number correctly and that you are able to disable your call waiting service.
If these things do not help, it is possible it is something with your modem or modem setup. First, make sure you have the correct drivers for your modem, and update them if necessary. Since each modem is different, you will have to consult the documentation that came with your modem for help on doing that. But, a good place to get your driver from is www.windrivers.com. If that doesn't help, it may be necessary to replace your modem.
Links to other information: U.S. Robotics Sportster 28.8 Modem Loses Connection Speed
Problems connecting at High Speeds with US Robotics 56k WinModem, LT Winmodem or HSP Winmodem.