A fax machine dialing in. Fax machines are notorious for doing this. Someone is trying to send a fax, but somehow sent it to the wrong phone number. Forward your line to your fax line for approximately ten minutes. Once the fax gets through, your telephone line will be cleared of this problem.
Chances are, it is call forwarded incorrectly.
A good give away is when the telephone can't receive calls, but can still make outgoing calls. Sometimes callers will complain of a busy signal when the handset is lifted to listen for a dial tone. This is a message indicator, and the phone is letting you know that there is a new message on voice mail. Once new messages are heard, the stutter dial tone will clear. The calls are probably going to another phone.