HOW DO YOU CLEAR ALL YOUR INFORMATION FROM AN OLD IPHONE?

I’ve upgraded to a brand new iPhone 3G. I have a ideally excellent initial era iPhone I wish to sell on eBay. How do I have certain all my personal report (including my phone number) is private from the old phone prior to I sell it? I synced the phone to remove my songs, apps, hit information, etc. But even yet the old phone isn’t activated, it still shows my phone number. Is there a approach to remove that?