If you are using a modem to dial up to the Internet you may get booted offline if someone attempts to call the line in use. A handy trick to avoid this is to disable call waiting before dialing up. It is usually a good idea to notify all members of the household before doing so.
To disable call waiting you need to input a series of numbers before your dial up number. You will need to verify with your phone company what these numbers are. Also double check to make sure there is no charge associated with the feature.