EDIT 16/06/2015 - Replaced Cellular Antenna - no fix.
I own a small IT repair business and we've recently expanded into the iPhone/iPad repair sector. A customer came in with an iPhone 4s, and the customer had replaced the screen for themselves. The issue is that the iPhone now can't be used for phone calls, texts or internet access. Instead of stating their provider in the top left of the screen it simply says "Searching...".
I have confirmed that it is not a SIM based error.