Hi, I'm working on an interactive voice response (IVR) system that will require PIN entry via keypad and will be accessed from GSM phones and GSM payphones.

How can I prevent PINs entered by the users from being accessed later from the phone/SIM card memory?

Can I ensure that the PIN numbers won't be saved in phone memory by having the user enter a certain number of digits via keypad before entering the PIN, so that the memory slot will be used up before the PIN is entered? What is the maximum number of digits that the phone will save?