Good luck with that. We have the same problem getting faxes directly into Athenanet. RingCentral doesn't have that option. I am manually forwarding every fax to the Athena number as they come in. Pretty crappy service.
RingCentral is VOIP service which is actually different from a digital line - in any case, for integration with an EMR you will have better luck getting a dedicated line through a provider such as eFax or Faxage. You can port your number to them if it's already widely known and in use.
That said- depending on your setup you should not need to manually forward each fax, you can set up a dummy user with appropriate call handling to forward the number to an external line. You may need to adjust settings on the line to designate it as a fax? I would get in contact with support and/or your account manager to troubleshoot your setup.