question

If this is happening only once in a while, they are probably telemarketers.  I get at least 3 of those calls a day.  When I call the number back, it goes to a dead line or I'm told all lines are busy.  These number are put into the AH file (also known as the blocked numbers).  If it is happening on numbers you know are good (customers etc), you might need some assistance in troubleshooting by opening a ticket.

USA-based acct | 2287 Users | 3800 DID lines | 28 sites | all Polycom VVX500 phone handsets 

2018-06-04  There is a condition in the RingCentral Service Web portal sometimes where a user has entered some advanced call rule in place to forward the number under some certain circumstances and it has been a case of the User forgetting this was setup last week before the weekend or before a vacation/absence.  

That would be where I would look to start your troubleshooting.

Next place in the portal to check is Message Handling - check forwarding is not setup to some other number.

Does the user have a Mobile App installed?  Do they have it set to Do Not Disturb?

Do you monitor them in RingCentral Desktop HeadsUpDisplay from your workstation?  If their status is a red dot - they may be "stuck" in Do Not Disturb and not realize it. 

The other place to review is the handset if Forwarding or Do Not Disturb is enabled on the handset.

RingCentral Support can also help you eliminate any RC portal set conditions that would cause this. 

Sometimes, as the folks above stated, it really is a computer generated robo-caller checking the number picks up. They then sell this as a "live" number to other spammers.