Generally, when your iPhone says it is searching for service, you are in an area where the cellular reception is not strong enough to make a call or send and receive data, and you won't be able to get service no matter what you do. However, on some occasions, your iPhone may be experiencing a minor glitch, and you may actually be able to obtain service despite the searching indicator. In this case, you can turn your iPhone off and back on to reset its signal detection.
Press and hold the power button on the top-right of the iPhone for five to 10 seconds or until the power-off screen appears.
Swipe the red power-off bar to turn the phone off. Wait for 30 seconds.
Click and hold the power button on the top of the phone again to turn the phone back on. The Apple logo appears, then the lock screen appears.
Swipe the bar at the bottom to unlock the phone. If your phone still says "Searching" instead of showing signal bars, you can try repeating the process, but you probably won't be able to get service.
Make sure your iPhone is updated with the latest software. A bug in earlier software caused the iPhone to search for service unnecessarily.