[John Jorsett]

Sometimes these things are fax machines that someone puts the wrong number into. If you hear a beep ... beep ... beep before it hangs up, chances are that's what's happening.

[Brad from Tennessee]

Your phone company may have a call blocking feature which will block calls from specific numbers.

[NerdDad]

Not a fax machine. My wife and I are both IT types and are familiar with the fax beeping.