A no service error can also occur among other random issues triggered by minor firmware glitches. If this is what triggers your iPhone to show a no service error, then a simple reboot or soft reset will likely fix it. Thus consider this as the very first solution to try, if you haven’t yet done it. Just follow these steps:

1. Press and hold both the Side and Volume Up buttons for a few seconds.
2. Release both buttons when the Slide to Power off bar appears.
3. Swipe or drag the bar to turn your device off.
4. After about 30 seconds, press and hold the Side button and then release when the Apple logo appears.