Another very common failure on iPhones is the classic searching or no service fault. Where the phone will constantly search for signal or show no service, even when there is no sim card installed!
This fault is related to the baseband in the phone and can be caused by many components.
A good way to test if this is a baseband issue is to dial *#06# and see if your imei number pops up on the screen. If not you are very likely to have a baseband issue.
In the iPhone 6 there is a weak spot under the baseband where traces often break, in the iPhone 7 the power management chip for the baseband fails, on all models shorts on certain lines can also cause this issue.
Most of these issues can be repaired as long as the baseband IC itself is not faulty, and with our experience we are able to diagnose and fix most searching/no service problems on iPhones.