Iphone Searching for service?

After I fixed an iPhone 4 for AT&T everything seems to work fine except it say searching for service then it will switch to no signal and back and forth. Is this because I have a non active sim in the phone or is there another problem? I know the connectors are fine I have check them a few times.

What did you fix? You forgot that part. And it doesn't sound like the sim since you are in. Maybe you can re-set up the phone. Best luck.