My phone suddenly says No Service — what to check, in order

First, read what the phone is telling you

The status bar distinguishes several states that people lump together as “no signal”:

The checklist, and why this order

Each step is ordered by likelihood × cost to check. An outage check costs you nothing and rules out the one cause you can’t fix; the resets take a minute and fix the most common cause; SIM, account, and software checks get progressively more involved. Skipping ahead to a factory reset — the internet’s favorite advice — costs an hour and is almost never the answer.

1. Rule out an outage. Check whether people nearby on the same carrier have service, or open your carrier’s status page over Wi-Fi. If it’s an outage, nothing on your phone will fix it — turn on Wi-Fi calling and wait.
2. Airplane mode for 30 seconds. Toggle airplane mode on, wait 30 seconds, toggle it off. This forces a fresh network registration and is the single most effective quick fix.
3. Restart the phone. A restart clears radio state that the airplane-mode toggle doesn’t reach. Yes, it’s cliché; it also works.
4. Select the network manually. In cellular/network settings, turn off automatic network selection, wait for the list, pick your carrier explicitly, then turn automatic back on. This dislodges a phone that latched onto nothing.
5. Check the SIM or eSIM. Physical SIM: power off, reseat it, inspect for damage, and test it in another phone if possible. eSIM: confirm the line is listed and enabled in settings; toggle it off and on. Never delete an eSIM profile as a troubleshooting step — only your carrier can reissue it.
6. Check the account over Wi-Fi. Log in to the carrier app or website and check the line’s status: billing hold, suspension, lost/stolen flag, or a number transfer in progress each cut service silently.
7. Update software state. Install any pending carrier-settings update and OS update, confirm VoLTE is enabled, and as a last resort reset network settings — this forgets saved Wi-Fi passwords.
8. Escalate to the carrier with specifics. Tell them when it started, where it happens, what the status bar shows, and everything you tried from this list. Ask them to check the line’s provisioning and whether the SIM needs reissuing.

Two of those steps open onto a page of their own when the one-line version isn’t enough: step five, on whether the SIM itself is the fault, and step seven, if the trouble started right after a software update.

If it only happens in one place

A phone that fails in one building but works elsewhere isn’t broken — you’re looking at coverage or construction. Indoor dead zones come from the building itself; a problem that appeared when you switched carriers is usually band mismatch; and Wi-Fi calling is the free fix that makes the location stop mattering for calls and texts. If the free fixes don’t hold and the signal is fine outdoors, the paid options are a signal booster or a network extender — and which of the two works depends on whether you have outdoor signal or home internet.

When it’s genuinely the carrier’s turn

If you’ve worked the whole list, the remaining causes — provisioning stuck on their side, a SIM that needs reissuing, a tower problem they haven’t flagged — are only fixable by the carrier. Two of them are worth separating before you call, because they need different conversations: how to tell an outage from a problem on your own line, and whether the account layer refused you, and which of five causes it was. Give them the exact history: when it started, where it happens, what the status bar says, and what you’ve tried. That single sentence of context skips the scripted first tier of support, because every item on it is something they’d otherwise walk you through again.