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.

Minor network errors can also be remedied by performing the Airplane mode trick. It works by simultaneously disabling wireless radios of your phone and then enable them back. Many iPhone owners who encountered relevant network issues have found wonders from this trick so it wouldn’t hurt if you’d try this too. Here’s how it’s done:

1. Tap Settings from the Home screen.
2. Select Airplane mode.
3. Toggle the switch next to Airplane mode to turn the feature on. All wireless functions of your phone are then disabled simultaneously.
4. While the Airplane mode is enabled, perform a soft reset on your phone.
5. Wait until your phone is finished rebooting and then go back to Settings-> Airplane mode menu.
6. Finally, tap the Airplane Mode switch again to disable the feature.