my first Iphone Issue

I did have one problem with my iphone, when I activated the phone I had full signal from the start, then for some reason towards the end of the day it kept jumping from 1 bar to NO SERVICE, i did everything, called apple, called AT&T and nothing even Resored it on itunes... end result I had to take it back to apple... and even tho i purchased it online the manager was so nice he did an instore no receipt return... i think Apple has such great Customer Service!
Well i now have a brand new one with full signal everywhere... very weird Iphone issue i thought...