I have an iPhone 4s in with damage to the logic board. During the microscope exam I discovered that the U18_RF chip is missing. It was apparently knocked off when the customer tried to replace her battery and pried the old one out of her phone using the logic board as the fulcrum. This is a common problem but can be avoided by only prying from the side where the battery is against the side wall of the phone.
The photo on the left is the tiny little chip (Next to a Dime) that is responsible for her phone constantly searching for service and never finding the tower. The photo on the right is where the chip needs to go.
After reflowing the solder on the logic board I used a reflow station to solder the tiny little chip onto the iPhone's logic board. That did it, the phone was able to find the service and the ability to place calls has returned.