Again, if all you want is the "ring multiple phones" service as you already have it, then, well, you already have it.

But that's not the same as the service you describe, giving the other phone a "number" under specific calling restrictions. As I described, that's where the technical problems start, and they are not as short as you seem to believe. It's doable, but not trivial (and therefore expensive to implement, for arguably marginal return). You can choose to not believe it, but that doesn't make it easier.