Such “erased” information usually remains stored in the phone’s memory while being removed only from the indexes in the phone’s operating system. But people with the right software can still recover it.
Scary: A security company recently bought 10 used cell phones on eBay. From the phones, they recovered bank account numbers and their passwords, confidential corporate business plans, private pictures, messages between a married man and his lover and thousands of pages of other private messages.
What to do: Some phones have special instructions for a “full” reset that erases everything in memory. Consult the manual or call the manufacturer.
If you can’t eliminate your private information, consider destroying the phone instead of selling it… giving the phone to a child or other family member (who doesn’t have the tools to recover your private information) to keep it from hackers… keeping the phone in your car or home for emergency use—deactivated phones can still call 911.