I have mentioned it here before that it is not safe to dispose of a “smart” cell phone because of all the personal data it can contain.
If you have an ANDROID phone you just may be in luck! Feel secure that no one is finding your personal data if they buy your old phone.
One of my Fav-o-rite newsletters is called Windows Secrets, and you have to be a member to read the whole article, but I will give you some highlights to see if this procedure might be useful to you.
Step 1: Back up the phone’s user files
Step 2: Stop/suspend synching and backup services
Step 3: Download and install a secure-erase tool
Step 4: Remove SIM; close wireless connections
Step 5: Remove optional memory cards, if present
Step 6: Clean all memory areas on the phone
Step 7: Encrypt your phone’s remaining contents
Step 8: Factory-reset the operating system
The final steps 9 and 10 are in the article, and there is a detailed description for each step with screenshots and helpful hints.
Or you can call me if you want some assistance with the process