Dial - Up Networking Help Line Noise Troubleshooting.
Line noise can have many sources.
Unplug answering machines, other phones from your phone lines and see if you still get the garbage characters when you dial up. Some devices can put a small drain on the phone line current and interfere with your modem transmission. If your modem is external, do the same for the cable connecting the modem to your computer. Wiggle the phone cable while you're dialed up and see if the garbage characters appear. If they do, you might need a new cable.