Dial-up connections are very sensitive to noise and interference. The cleaner the phone line, the better your connection is going to perform. Check your phone line for interference by picking up a telephone, dial one number, and listen to the receiver. There should be total silence. If you hear any hissing, popping, or other noise, report the issue to your telephone company to have your line checked for trouble.
Also, some phone services can cause problems with dial-up connections. Digital phone lines offered from cable TV bundles may not work very well with dial-up. Services such as call waiting may need to be disabled if your modem is not compatible with the tone that an incoming call puts on the line (in most areas, you can dial *70 before the phone number to disable call waiting).
You can stabilize your connection by using an initialization string on your modem.
See article: Stabilize A Dial-up Connection With an Initialization String