[gmmour]

It's not a matter of the billing system getting "confused", trust me the billing system can handle things very easily, moreover, NZ only has two providers and this makes things a lot easier! It is a matter of subscribers not getting enough information (which is their right) on what they're paying for each call.

Here in Greece, although the in-network plans are limited (sometimes even to subscribers of the same network with a special subscription), here's the solution they've found.

There is a short code where you can send an SMS with your desired number for free and you get a reply SMS which tells you which network the number you want to call belongs to. For offers that are valid only to specific subscribers of a specific offer, there is another free SMS short code which tells you if the subscriber you want to call is part of the offer.
The local regulator (EETT.gr) has actually forced operators (both fixed and mobile) to provide information on the carrier each number belongs to, free of charge. And that's because each fixed carrier charges different rates for calls to different mobile networks and mobile providers (although their minutes are allover) sometimes have special reduced rates for in-network calling. If their tarrifs make no distinction between destination networks then they're not obliged to offer this kind of information to their subscribers!

Moreover, when you get your (fixed or mobile) call statement, they're obliged to give you the name of the provider which the number belongs to, next to the called number. That's a clever solution.

But anyway, there is no excuse in not offering MNP just because it would cause some initial confusion to the subscribers. MNP will actually bring a lot more benefits than the initial confusion that will be caused (like here in Greece where tarrifs are network independent).