Why do I have no network coverage?

Why do I have no network coverage?

I'm pretty sure the phone is properly unlocked and i put my sim card in and activated it on my cell phone plan and no matter where I go (places where on other phones i get great coverage) i get no signal. Is this an issue with the phone, the carrier, unlocking, or just a bad antenna? what do i do to fix the problem? would i just be better off buying another one since i may have a faulty handset?