[quote name='J7.']If you delete numbers one at a time isn't that data still retrievable? Some of the phones are so old they don't power on.[/QUOTE]
Unless you are a celebrity, using phone for criminal activity, or you are dealing with HIPAA just recycle it. Your data isn't worth it for someone to go through the trouble. I am sure your phone company is sell more personal information than anything someone will get from your phone.