This problem is typical of a poor connection, possibly a noisy phone line.
Pick up the phone (without the computer connected), dial a single digit (to stop the dial tone for a short time) and listen to see if the line is silent. There should be no sound, static or clicks whatever.
Pick up the phone (without the computer connected), dial a single digit (to stop the dial tone for a short time) and listen to see if the line is silent. There should be no sound, static or clicks whatever.