Idea

If a Skype caller is calling your mobile or landline phone, no number will be displayed on your phone unless they previously set up Caller Identification. Having said that, it doesn't announce or have any identification that it is a Skype caller. At best it would show as a VOIP caller.

Playing devil's advocate, how would RingCentral or ANY OTHER telephone service provider know that it's a Skype caller and that you want them blocked? As far as I know (I'm not a VOIP technician or engineer) that simply isn't possible.

You can, however, block all Private or Unknown callers so that if a Skype caller who hasn't set up Caller Identification does call you, it will indeed be blocked.