By law you can port a post paid number to another network on the condition that it has not been suspended for non payment. The networks don't like this and, wrongly, insist that you migrate to prepaid before releasing the number. That way you have to pay up your account before migrating to prepaid. They can then milk you for airtime/data while the porting takes place. Often there are "problems" porting out and you have to keep topping up airtime/data never knowing if tomorrow is your lucky day or not.