I need help! (Cell Phone Question!)

I currently have an iPhone.
I ordered a new SIM card to put into a different phone with the same number, from AT&T because my iPhone is cracked to all hell and I can't afford to have it fixed... blabbity blah blah...
I got the Sim card in the mail today, I saw a page on the AT&T site to activate, but I don't know if since I'm already customer if I have to call to activate this other phone. Sooo help, please?

I'm technologically hopeless.
Thanks

Yeah I'm pretty sure you have to call and activate....not sure sin rim thru verizn and mine doesn't have a sim card