Phone harassment

The phone's ringing several times then hangs up before a friend calls and leaves a message.  This has been going on for the past few days.  They ring the phone once, then it rings three times, then it rings once, then again twice, and this goes on about six times.  Right after that, a friend will leave a voice message.  Isn't that strange?  As if the ops know that a friend is about to call me.  But how do they know unless they are scanning that person somehow, too?  Or putting the idea in the friend's mind to call me.  I don't know how this works but it is no coincidence.
-suz