The SIM contains another number which is recognized by the Mobile Operator and then the SIM is given the phone number.
I am not 100% sure but I think you can change your number without changing the SIM, right ?
When you want to check your code what is the easiest way to get an overview of what is going on. For example the vlaues of variables the status of loops. I want to have a quick overview and see if everything is done as i ecpect it.