3G won't work...

[Iphone4_stl]

Whenever I am using wifi, everything (safari, facebook, apps, etc) works just fine. When I'm using my 3G data, nothing will load at all. Best buy gave me a new phone ( this is my 2nd this week). Before leaving best buy, I tested the 3G to make sure that it worked, which it did. I got home and synced it up to my iTunes and let it download my contacts and everything else. Later on, I try the 3G, and it doesn't work. Any idea what is wrong?