Fix iPhone Wi-Fi Calling After iOS Update: No Service

You updated your iPhone last night and this morning Wi-Fi Calling is either greyed out, stuck on "Activating," or throwing a "No Service" banner even though you're sitting two feet from your router. Annoying, and it's almost never your carrier's fault.

The fix (do this first)

Force the carrier bundle to re-provision. The iOS update ships a new carrier settings file, and sometimes the old one stays cached. Here's the sequence that works on iOS 17.4 and 17.5:

Open Settings > General > About. Wait 10 seconds. If a "Carrier Settings Update" popup appears, tap Update. This alone fixes it for a lot of people.
Now go to Settings > Cellular. Tap your primary line (the one with the phone number).
Toggle Wi-Fi Calling off. Don't skip the wait — leave it off for a full 30 seconds so the IMS registration on your carrier's side actually times out.
Toggle it back on. If it asks for your emergency address again, that's a good sign — it means re-provisioning is happening.
If it sticks on "Activating..." for more than 60 seconds, kill the Settings app (swipe up from the bottom, flick it away) and reopen it.

Still dead? Reset just the network stack, not the whole phone:

Settings > General > Transfer or Reset iPhone > Reset > Reset Network Settings

You'll lose saved Wi-Fi passwords. You won't lose photos, messages, or anything else. After the reboot, reconnect to Wi-Fi and turn Wi-Fi Calling back on.

Why step 1 matters more than it looks

What's actually happening here is that Wi-Fi Calling isn't a phone feature — it's an IMS (IP Multimedia Subsystem) registration with your carrier. Your iPhone talks to a carrier server, hands over your SIM identity, and gets back a token that says "this device is allowed to route calls over the internet." That token has a TTL. When iOS updates, the carrier bundle that defines how your phone requests that token gets swapped out, but the cached registration doesn't always get invalidated.

So you end up with a phone that thinks it's registered and a carrier that thinks it isn't. The "No Service" banner appears because the phone stops trusting the cellular radio for voice and hasn't successfully established the Wi-Fi fallback. The fix in step 1 forces the handshake to happen from scratch.

The reason step 3 works is the 30-second wait. Toggling too fast means the carrier never sees the deregistration, so the next registration gets rejected as a duplicate. I've watched this on T-Mobile and Vodafone eSIMs specifically — the deregistration packet needs to round-trip.

If it still won't activate

Some less common variations:

eSIM line stuck at "Activating." Remove the eSIM and re-add it. On iOS 17.4 there's a known issue where the eSIM's IMSI gets orphaned after an OS update. Settings > Cellular > tap the line > Remove eSIM, then re-scan the QR from your carrier's app or portal.
Wi-Fi Calling works on cellular data but not Wi-Fi. Your router is blocking the IPsec ports (UDP 500 and 4500). This hits people on locked-down corporate networks and some ISP-supplied routers with "SIP ALG" enabled. Disable SIP ALG in the router, or test on a phone hotspot to confirm.
Only one line fails. Dual-SIM users see this a lot. Wi-Fi Calling has to be enabled per-line, and after an update the secondary line often reverts. Check both lines individually under Cellular.
"Contact carrier" error. That means the IMEI isn't whitelisted for Wi-Fi Calling on your account. This genuinely requires a carrier call — nothing on-device fixes it. Ask them to re-provision the IMEI on the line.
Beta iOS build. If you're on a developer beta, carrier bundles lag behind. Rolling back to the public release usually restores it within an hour.

Preventing this next time

Before you tap "Install Now" on the next iOS update, do two things. First, check Settings > General > About and note your current carrier settings version. Second, after the update completes, go back to that same screen and confirm the carrier version changed. If it didn't, force it by toggling Airplane Mode on for 15 seconds and off again — that triggers a carrier settings check.

Also worth doing: leave Wi-Fi Calling enabled permanently rather than toggling it when you travel. Constant on/off abuse is what creates orphaned registrations in the first place. If you're worried about battery, Wi-Fi Calling uses less power than a weak cellular signal, not more.

And skip the "reset all settings" advice you'll find on carrier forums. It nukes your preferences and rarely fixes anything that the network reset doesn't. The real fix is almost always re-provisioning — you just have to force it.