Phantom Calls - Odd number in Call Display

This is a worldwide scourge and there is nothing in particular RingCentral or you can do about it, unfortunately.  You can read about US legislation around the STIR/SHAKEN protocols that are being touted as a possible solution.  Another possible solution is that "normal" phone calling will just die away to be replaced with things like What'sApp etc because people will get so annoyed they will stop using regular phone numbers.

Sounds like a SipVicious attack (Port scanning on SIP ports).  Have your IT consultant tighten up your firewall and to only open the RingCentral ports for their IP address ranges.  You can confirm this by checking the Call logs....if these calls aren't there than most likely SipVicious attack.