To make an automated process of creating user without human interaction, you need a custom application which will call RingCentral API CreateUser and fill the value automatically and randomly from a predefined value.
But the problem I see here is the CreateUser API has emails parameter as mandatory field.
If you just want to create an extension, as you said and will not assigned to user, you can define your custom application to use CreateExtension API and here I don't see email as a mandatory field.
We have that same issue - not a human specific phones in area. (reception, break room, etc.)
We just manually added the user extension - giving it a password/pin - so to not have to send the activation email - and used a 'general bucket' non-email email address. (gets sent to a catch-all email address - but not monitored)
Only trouble with RC is that any phone that rings, has to have a digital account (meaning a DID license assigned)
Hope that helps.