iPhone “SIM Not Supported” – Fixing Carrier & Locked-Phone Issues in the UK (Full Guide)

If you’ve just inserted a new SIM into your iPhone and suddenly see the dreaded “SIM not supported” message, you’re not alone. This error is very common in the UK — especially when switching from EE to O2, or popping in a cheap SIM-only plan from MVNOs like giffgaff, SMARTY or VOXI. I’ve dealt with this issue multiple times while testing SIMs for AvNexo, and it usually comes down to carrier locks, old activation profiles, or a mismatch between UK network settings.

Below is the most complete UK-specific guide you can follow to fix it safely — without wasting time on random YouTube “tricks” that don’t work.

1. Confirm If Your iPhone Is Carrier-Locked (Most Common UK Cause)

In the UK, older iPhones sold via EE, Vodafone and Three (especially pre-2022 devices) were often network-locked. If your phone is still locked, any SIM from another carrier — including cheaper MVNOs — will trigger the “SIM not supported” message immediately.

How to check the lock status:

  1. Go to Settings → General → About.
  2. Scroll to Network Provider Lock.
  3. If it says No SIM restrictions, your phone is unlocked.
  4. If it says anything else — it’s locked to a specific UK carrier.

Quick note: If the device is locked, it will reject new SIM profiles, including those from budget networks that rely on bigger carriers. For reference, here’s our Connectivity & Network Hub where you can check how different UK SIM profiles behave across networks.

2. If the iPhone Is Locked – Request an Official Unlock

The good news: UK networks must unlock your device for free (as long as it’s fully paid). I’ve done this many times — usually it takes minutes on Three, a day on O2, and sometimes 48–72 hours on Vodafone.

How to unlock by carrier:

  • EE: Automatically unlocks after contract ends. If not, call 150.
  • O2: Supports instant unlocks via the “MyO2” app.
  • Vodafone: Unlock via their online “IMEI unlock form”.
  • Three: Most models sold after 2020 ship unlocked, but older ones can be unlocked via their chat support.

You’ll need your IMEI: dial *#06# on the Phone app.

Avoid third-party unlocking websites. Many are UK-based but simply resell the same official unlock request — and sometimes never complete the process.

3. Check If Your iPhone Is Showing “SIM Not Supported” Due to Activation Issues

Sometimes the phone is actually unlocked, but the iOS activation system gets stuck — especially when switching between physical SIM and eSIM. I’ve seen this plenty when testing SIMs from iD Mobile and BT Mobile.

Try a full activation refresh:

  1. Go to Settings → General → Transfer or Reset → Reset → Reset Network Settings.
  2. Insert the SIM again.
  3. Connect to a strong Wi-Fi network.

The connection is important: the activation server must confirm your carrier settings, and UK broadband (fibre or FTTC) tends to be more stable for this than mobile LTE/5G.

4. Install the Correct Carrier Settings

Each UK network — including MVNOs — uses unique carrier profiles. If they’re missing or outdated, you’ll see SIM errors, unstable signal, failing calls, or no mobile data.

To update:

  1. Insert the SIM.
  2. Connect to Wi-Fi.
  3. If prompted for a Carrier Settings Update, tap Update.

You can also check manually:

Settings → General → About If an update is available, you’ll see a pop-up instantly.

I’ve noticed this especially when testing MVNOs like SMARTY (on Three) and VOXI (on Vodafone). Their profiles often require a reboot.

5. Fixing the Issue When Using an MVNO SIM (giffgaff, SMARTY, Lebara, VOXI)

These networks piggyback on major UK networks and occasionally trigger the “SIM not supported” alert due to APN or activation timing differences.

Here’s what usually works:

  1. Insert the SIM and leave the phone locked for 2–3 minutes.
  2. Reboot your iPhone.
  3. Connect to broadband (NOT mobile data).
  4. Wait for the activation popup — don’t exit Photos or Messages until it appears.

Also ensure APNs match what the network uses. You’ll find APN details in our Network Hub if you need them.

6. When You Bought a Second-Hand iPhone (The UK’s Biggest Hidden Problem)

The UK second-hand market (Facebook Marketplace, eBay, CeX, MusicMagpie) is notorious for locked devices being sold as “unlocked”.

If the iPhone is locked to a foreign carrier:

You may see:

  • AT&T (US)
  • Verizon (US)
  • Sprint (legacy)
  • SoftBank (Japan)
  • KDDI (Japan)

These devices usually fail activation entirely in the UK and cause the “SIM not supported” error on every network including EE, O2, Vodafone, and Three.

If this is the case, ask for a refund immediately. Many overseas-locked phones can’t be unlocked through UK carriers.

To avoid this in the future, see the advice in our Security & Privacy Hub, especially the section on checking IMEI before buying.

7. Check eSIM Conflicts (Very Common on UK iPhones)

When switching from eSIM to physical SIM or vice versa, iPhones can get confused about which SIM slot should be active.

Fix:

  1. Settings → Mobile Data
  2. Disable any old eSIM profiles
  3. Reboot iPhone

On dual-SIM iPhones, make sure your physical SIM is set as the “Primary line”.

8. Update iOS (Activation Errors Spike on Outdated Versions)

Some iOS versions have bugs where new SIMs won’t activate until the phone is updated. I saw this when testing iOS 16.5 with VOXI — it just refused to activate until upgrading.

  1. Go to Settings → General → Software Update
  2. Install the latest version

After updating, insert the SIM again.

9. Try the SIM in Another Phone

This step is underrated — but very telling.

  • If the SIM works in another phone → your iPhone is locked or misconfigured.
  • If the SIM doesn’t work anywhere → the SIM itself is unactivated or faulty.

New UK SIMs from Three, Vodafone or MVNOs sometimes need an online activation form completed before they work.

10. Contact Apple Support (When the Error Is Hardware or Baseband Related)

Apple can check activation logs and baseband firmware. If your phone had recent liquid damage or a failed update, the modem firmware may be corrupted — leading to this error.

Visit: Settings → General → About If the Modem Firmware field is blank → baseband failure → only Apple can repair it.

In the UK, you can visit Apple Stores or authorised service providers like KRCS and Stormfront.

Conclusion

The “SIM not supported” error on an iPhone in the UK almost always comes down to one of four things: carrier locks, incomplete activation, outdated network settings, or using a foreign-locked device. By checking your lock status, installing carrier updates, resetting network settings, and confirming SIM activation, you can resolve almost all cases without expert help. And if it turns out the phone is locked, UK carriers must unlock it for free — just request it officially.

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