In my country we have one method that is kind of stupid but it should work. You pay your phone bill after the deadline so you are always one month behind. This

In my country we have one method that is kind of stupid but it should work. You pay your phone bill after the deadline so you are always one month behind. This way you are always in debt and as such you can not transfer your number to another operator. I didn't test it but it should work since at least for my operator.