True its voluntary, but how do u effectively consume off credits if you are planning to port. And if you need to pre-plan excessively for a port whereby plans are so dynamic... it becomes an effective tool to lock u to them or allow them to earn more than they ought to have.Topping up is voluntary.
But they would have to build in more safeguards if change the business model more towards a use first pay later.
How much is the cut off for excess charges to cc for example.
Roaming burst usage how?
It's likely a strategic tool used to prevent people from porting over AND/or earn in the process