hi there, iphone 4, no service, handset problem

my friend got a iphone 4 and when the a sim is placed in2 this phone it comes up as no service, i even put my own sim outa my iphone and this still happens so that rules out the carriers fault, obviously a problem with the actual handset, as when they place their sim in2 my phone it works. the phone is the problem, as it is still under all the warrantys, does this mean apple will replace the handset there and then when i bring it in?

any help would be great