Clients are calling leaving voicemails-phone never rings and we never get voicemails

Since we have started with ringcentral, we have had at least three customers tell us (via email or in person) that they have called and/or called and left messages. However, there are no records of the calls (if we were in the office at the time, the phone didn't ring and there is no record in the online call log or the phone call log). There are no records of the voicemails either on the phone or on the online call log. Some of these calls have been from people trying to pay us for their outstanding bills.