Check that it is not the network
Look at another phone on the same carrier in the same place. If it has no signal either, there is an outage or no coverage, and the fix is waiting or moving. Carriers publish outage maps, and a search for the carrier's name and "outage" usually finds it within minutes.
SOS or SOS only means the phone can see a network but your carrier's is not available — it can still call emergency services through another one. That points at coverage or the account rather than the phone.
Toggle Airplane Mode
Turn Airplane Mode on, wait fifteen seconds, and turn it off. That forces the phone to register with the network again, which is exactly what a phone stuck on No Service after a flight, a tunnel or a border crossing needs.
If you have just travelled abroad, check Settings → Cellular → your line → Data Roaming. Calls can work while data refuses to, or the reverse.
Restart, then check for a carrier update
Restart the iPhone. Then open Settings → General → About and wait a few seconds: if your carrier has published new settings, a prompt to update them appears there. It installs in seconds and is a common fix for a phone that lost service after the carrier changed something.
Updating iOS itself is the other half — our update guide covers doing it over Wi-Fi when cellular is the thing that is broken.
Check the SIM or eSIM
Settings → Cellular shows each line. A line marked as off, or a message such as No SIM or SIM not supported, tells you where the problem is. For a physical SIM, eject the tray, check the card is seated flat and clean, and put it back; try it in another phone if you can.
SIM not supported on a phone that worked before usually means the phone is locked to a different carrier, or the SIM has been deactivated — both are for the carrier to fix, not the phone.
Reset network settings
Settings → General → Transfer or Reset iPhone → Reset → Reset Network Settings. It clears the cellular configuration along with Wi-Fi networks and Bluetooth pairings, and rebuilds it from scratch. Your eSIM is not removed by this — only by the separate Erase All Content option, which you should not use here. Our network reset guide covers it.
On a beta, a lost signal is a known kind of fault; our beta cellular guide covers that case.
Call the carrier
If another phone works with your SIM, or your SIM works in another phone, the account and the network are fine and the iPhone is the problem. If neither works, the carrier can see things you cannot: a suspended line, an unpaid bill, a reported-lost SIM, or a fault on their side. Calling from another phone is faster than any further step here.
When it is hardware
A phone that shows No Service everywhere, with a SIM proven to work in another phone, after a network reset and an iOS update, has a fault in its cellular radio or antenna. Drops and liquid are the usual causes, and only a repair fixes it.
Frequently Asked Questions
What does SOS mean on my iPhone?
Your carrier's network is not available where you are, but the phone can reach emergency services through another network. It is a coverage or account issue, not a fault.
Why does my iPhone say No Service?
An outage or no coverage, a SIM or account problem, or a stuck network registration. Toggle Airplane Mode, restart, and check another phone on the same carrier.
Will resetting network settings delete my eSIM?
No. Reset Network Settings keeps your eSIM. Only Erase All Content and Settings can remove it, and you should not use that to fix a signal problem.
Why does my iPhone say SIM not supported?
Usually the phone is locked to a different carrier, or the SIM has been deactivated. Both need the carrier; the phone itself is fine.