Can a phone plugged into the line hinder my modem?

A phone plugged into the line can cause problems with your modem, as some phone ringing devices have low enough impedance to interfere with normal modem transmission, especially at higher speeds. (Because the devices are wired in parallel, not in series, a device with higher impedance would cause less interference.) Unplugging the phone from your modem can decrease transmission errors.

The presence of several phones on the same line can also cause problems. Errors such as carrier detect loss, unusually slow response or transmission rate, and inability to connect at the desired speed can all be caused or aggravated by the presence of a phone on the line.

Also see:

• How does call waiting affect a modem connection, and how can I deactivate it?
• What is flow control, and how do I set my modem to use it?
• At IUB, what should I do if I'm having problems using my modem with my office telephone?
• What should I do if I am experiencing problems when I make a modem connection?
• What should I do if my modem gives me a "No dial tone" error?
• With a modem, why is my connection speed slower than the advertised speed?