Not receiving inbound calls from landlines

We have a number of clients and prospects that call us from traditional landlines.  

We have discovered recently that many of them are unable to get through to our Zoom phone service.  They are either just disconnected, or they receive a message that says they cannot be connected.  It happens across multiple landline carriers both intra and interstate.  We receive no indication that these calls were ever attempted.  VOIP callers and cell phone callers have no problem.  Anyone else having this problem?  Anyone found a fix?

Thanks.

Eric