The dial-up software often has options to help do this for you. For more information about these calling services, check the phone book or call the phone company. Just as Call Waiting can interfere with your modem connection, so can having someone pick up another extension in the house. Another issue in home connections is line noise. Data transmissions are much more sensitive to line noise than voice calls.
Line noise can result from bad weather, interference, and other conditions.
When using your modem, it can cause slow connections or even dropped connections. If this is a continual problem, you may wish to have your phone lines checked by a phone professional. Sometimes I call and can't get connected. If the phone line is busy, all available phone lines are in use. You may need to just keep trying.