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How to delete cell phone memory
Back up your contacts.
Before wiping your phone, you may want to back up your contacts list. You'll be able to restore this list after you finish the reset procedure.
If you're logged into your phone with a google (android) or icloud (iphone) account, the chances are good that all of your contacts are backed up in the cloud already. See back up an android phone on the google cloud for instructions on backing up your android contacts.