That can't be true. If a number was ported from Carrier X to Carrier Y and someone makes a call originating from carrier Z, I am pretty sure carrier X is not in

That can't be true. If a number was ported from Carrier X to Carrier Y and someone makes a call originating from carrier Z, I am pretty sure carrier X is not involved any more in the routing of the call, which means that carrier Z has some way to figure out which carrier to route to (Y in this case). So it's not just those two companies involved. At least in India, there exists a central database of ported numbers which every carrier uses to do the routing.