How do you clear all your information from an old iPhone?

I’ve upgraded to a new iPhone 3G. I have a perfectly fine first generation iPhone I want to sell on eBay. How do I make sure all my personal information (including my phone number) is removed from the old phone before I sell it? I synced the phone to remove my songs, apps, contact information, etc. But even though the old phone isn’t activated, it still shows my phone number. Is there a way to remove that?

Restore it. That would clear the data & anything from it.
For your phone number, just take out the SIM. That’s it.

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