Believe it or not: iPhone "Restore" does not delete personal data! - A Warning from CNET
When got a new smartphone how do you deal with your old iPhone? Surely most people would resell or give away their old iPhone, but you would be hesitated when you see the warning above. Don't be worry anymore, here is an easy way to solve this.
Erase All Data & Settings on Your iDevice
Contacts, SMS Text Messages, Call Logs, Photos, Videos, Voice Memos, Calendar, Notes, Reminder, Email, Safari Bookmark, iTunes DB, App (and generated data), System setting, Apple ID, Keyboard Cache, Passbook, etc.
Erase Traces of Deleted Files, Making Them Unrecoverable
Apps, Contacts, Messgaes & iMessage, Call Logs & Facetime, Calendar & Reminders, Notes, Voice Memos, Safari Bookmark, Photos, Videos, etc.