Can you activate a used iPhone online?

I'm purchasing a iPhone 4 8gb from my moms work. It was a spare they had and it was activated under a number but that is it. Can i activate it online or do you have to go to the store?

You should probably just go to the store, its loads easier.

You'll have to go to the store if you plan on using the same company. If you want to switch providers you'll have to get it unlocked by someone. The can usually be done by people selling phone cases at mall kiosks