Noise caused by devices on the telephone line. One brand of phone, the TouchPhone 200, causes a great deal of hum on the line. Unfortunately, Telstra will only guarantee a stable data connection over a standard telephone line of 2400 baud. Try disconnecting all devices from the line, including FAX machines, answering machines, portable phones, and other telephones. Disable Call Waiting.
Electric fences are a common source of interference.
Radio towers and CB radios nearby can cause problems.