That very much depends on whether the traffic between the two networks is equal. If it's very asymmetric one side can take advantage of the situation. Also keep

That very much depends on whether the traffic between the two networks is equal. If it's very asymmetric one side can take advantage of the situation. Also keep in mind that in some locations, free roaming is mandated by law so carriers have no choice but to offer it - in this case visited carriers can take advantage.