I can't confirm in practice, but, I suspect that, as it is not a two way conversation, a page, would probably be considered as one "call", if that.
It would not make sense to have, say, a four licence system, with perhaps 10 extensions, and only be able to page four sets at one time. And... that would only be, if no one else was engaged in a call.
I'm sure that another member has some practical experience with this.