It is difficult to log in to a shared VM only extension. Weirdly, you have to log in to them through a very specific method;
If you use any app (mobile, or desktop), or a browser to go to say "ringcentral.com" and click 'log in-> RC app' or "login.ringcentral.com" and login with your main number, VM Only extension number, and the password for that extension you get an error page that states "Sorry, you currently don't have permission to access this service. Contact your administrator to change permissions."
However, if you take the exact same login steps but start by going to "service.ringcentral.com" you can log in to the VM only extension just fine. The login pages look identical, and all will let you login successfully to a normal acount, but the do NOT work the same for VM Only extensions! Some flag or something is being mishandled.
The following URLs are the full lines I get once redirected:
INCORRECT METHOD (go to "ringcentral.com", click "login", Click "RingCentral App") Redirects to the following:
WORKING METHOD (type "service.ringcentral.com") Redirects to:
Note they go to very different login.ringcentral.com urls ultimately.
Hope this helps?