If your iPhone says "SOS Only" or "No Service", it can't connect to the cellular network. You can still make emergency calls, but regular calls, texts, and mobile data won't work.
Quick fixes to try first
Toggle Airplane Mode on, wait 10 seconds, toggle off
Remove and reinsert SIM card (or restart for eSIM)
Settings → General → Transfer or Reset → Reset Network Settings
Check Settings → General → About for carrier update
Update to the latest iOS
If quick fixes don't work
Software causes
Corrupted carrier settings
Network misconfigured
Carrier account issue (unpaid bill)
SIM card expired or damaged
Hardware causes
Antenna flex cable damaged
SIM card reader corrosion
Baseband IC chip failure
Water damage to RF components
Quick test: insert a known-working SIM from another phone. If service returns, the problem is your SIM or carrier account — not the iPhone. Check IMEI at Settings → General → About. Blank IMEI = baseband chip failure.
If none of the above works, the issue is most likely physical hardware damage — a failing chip, corroded connector, or broken internal component. If you've tried everything above and the error keeps coming back, the problem is most likely physical — something broken inside the device that software can't fix. That's when it needs professional hands-on inspection.