I keep an old PAYG as a spare, to top up I have a C/card sized card (does not show Phone number) which I pass to the Cashier (S/market) along with a tenner she sticks card in machine, job done.
One other forum suggested w"Text NUMBER to 2020". I make no claim for it though. I think many solutions depend on the service provider you're with, as they match your SIM number to your phone's IMEI. It's their database you need to access.