RC Labs - Volume/Campaign SMS

Saying "account" do you mean user extension?

In RingCentral terms, account = company(employer); extension=user (employee)

An extension can own multiple phone numbers and that user can send SMS from their own phone numbers.

If a phone number is not assigned to a user extension, the number is a company number. And a user who is assigned to handle call handling under the com[any IVR, will have the right to send SMS from that company number.