“Locked SIM” can mean two different things, and the fix depends on which one you have. A carrier (network) lock is tied to the handset and limits which provider’s network it can use. A SIM PIN protects the SIM card itself and has nothing to do with which carrier you use. A “No service” message on its own proves neither.
The quickest check on an iPhone is Settings > General > About. On other phones, the most reliable approach is to see exactly what message appears when you insert a different carrier’s SIM, then confirm with the carrier that sold the device.
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Carrier lock vs. SIM PIN: which one are you dealing with?
| Type | What is locked | Typical sign | Who resolves it |
|---|---|---|---|
| Carrier / network lock | The phone, restricted to the provider that supplied it | A message that the SIM is not supported or incompatible, or a network-unlock prompt, after inserting another provider’s SIM | The original carrier or retailer |
| SIM PIN | The SIM card | A prompt asking for a PIN when the phone starts or the SIM is inserted | You, using the PIN issued with the SIM; otherwise the SIM’s carrier |
| Blocked SIM (PUK) | The SIM, after repeated wrong PIN entries | A prompt asking for a PUK code | The SIM’s carrier |
Samsung’s UK support pages draw this same line: SIM locking is a SIM PIN feature, while a handset first supplied by one provider may be network-locked when used with another provider’s SIM.
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Method 1: Check the phone’s settings (iPhone)
- Open Settings > General > About.
- Scroll to Carrier Lock or Network Provider Lock. The label varies by region.
- Read the status. Apple Support (UK) states: “If “No SIM restrictions” appears next to Network Provider Lock, your iPhone is unlocked.”
Any other status means the iPhone is restricted. “No SIM restrictions” describes the lock state only. It does not guarantee that every carrier, band, plan or eSIM will work on the phone.
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Android menus differ by manufacturer and software version, and there is no single path to quote. If your phone doesn’t clearly show a lock status, use Method 2 or 3.
Method 2: Try a SIM from another carrier
This is a diagnostic test, not a final verdict. AT&T’s guidance for its devices is to insert a non-AT&T SIM. If the phone reports that the SIM is incompatible with the AT&T network, the device is still locked under that carrier’s procedure.
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- Power the phone off and remove your current SIM.
- Insert an active SIM from a different provider.
- Power on and note the exact wording of any prompt.
Interpret the result carefully:
- Carrier-lock or “SIM not supported” message: the handset may be carrier-locked. Confirm with the carrier.
- Normal signal and the ability to make a call: a good sign the phone accepts other carriers.
- No service with no lock message: not conclusive. The cause could be network compatibility with your device model, an SIM that isn’t activated, an account or service-status issue, or coverage.
Use a SIM you trust, and make sure it is activated before you draw conclusions.
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This is the only route that gives an authoritative answer and the only way to get the lock removed. Apple says only the current carrier can unlock an iPhone for use with another provider. Samsung directs network-locked users to the original provider or retailer. AT&T offers its own eligibility and request route, and T-Mobile directs customers to its account and support channels to check status.
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Eligibility rules and timing depend on the carrier, device, account, where you bought the phone, and your country. AT&T and T-Mobile policies apply to those US carriers, and the Samsung and Apple pages referenced here are UK support pages, so check your own carrier’s current terms.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What each prompt means and what to do
The phone asks for a SIM PIN
This is the SIM’s own security lock, not a carrier lock. Enter the PIN supplied by your carrier. Samsung notes that the default PIN depends on the provider and may be printed on the original SIM packaging. Don’t guess: repeated wrong entries can block the SIM. If you don’t know the PIN, contact the carrier.
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The phone asks for a PUK
The SIM is blocked. Ask the SIM’s carrier for the PUK through its official process, and don’t guess. The number of attempts allowed before further consequences varies by carrier, so check with yours rather than relying on a general figure.
“SIM not supported,” “network unlock” or similar after inserting another SIM
The handset may be carrier-locked. On an iPhone, confirm under Settings > General > About. On other devices, contact the original carrier or seller.
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“No SIM,” “invalid SIM” or “No service” with no lock wording
Treat this as a general fault first. A SIM can be inactive, damaged or unsupported, and the phone may have an account, activation or network-compatibility problem. Reseat the SIM, test it in another phone if you have one, and ask the SIM’s carrier to check its status.
Before you pay for an unlock or new hardware
- Identify the exact on-screen message first, as the table above shows that each message points to a different fix.
- Go to the carrier that sold or locked the device before trying any third-party unlock service.
- No special tool or accessory is needed for this diagnosis beyond a SIM and, if your phone requires one, the tray ejector it shipped with.
Carrier policies, device menus and procedures change. Samsung’s UK page was last reported as updated on 5 June 2026, and the carrier pages cited here were checked in October 2026, so confirm current terms with your provider.
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