SIM card not working: what to check, and in what order

Why is my SIM card not working?

A SIM card not working usually means one of three different faults, and the fix depends entirely on which one you have. Either the phone can't see the card at all ("No SIM", "Invalid SIM", "SIM card not detected"), or it sees the card but won't use it (a PIN or PUK prompt, or a carrier-locked phone), or it uses the card and still gets no service or no data.

The first is usually physical. The second is codes and locks. The third is almost always the account, the network settings or, abroad, registration. Read the message on screen carefully, because it tells you which of the three you're in.

We read Apple, Google, Samsung and Verizon support pages on 9 November 2026. This is desk research rather than a lab test.

iPhone says No SIM or Invalid SIM

Apple's own troubleshooting order is short and sensible, and it starts with the boring things:

1. Check that your plan is active. A suspended or unpaid line produces the same alert as a broken card.
2. Check the carrier lock. Go to Settings > General > About. If the iPhone is unlocked, "No SIM restrictions" appears next to Carrier Lock.
3. Restart the iPhone.
4. Check for a carrier settings update in Settings > General > About.
5. Reseat the SIM. Take it out of the tray, put it back, and make sure the tray closes fully. Apple warns that a tray from a different model or brand may not fit properly.
6. Try another SIM, or ask a carrier store to test with one. Apple's words: "You might need to replace your SIM card."

If the phone uses an eSIM rather than a card, Apple's advice is simply to contact your carrier. If the alert survives all of this, the phone itself may need service. Our guide to removing a SIM card from an iPhone covers the tray, and the US models that don't have one.

SIM card not detected on Android

Google and Samsung agree on the order, with different menu names.

On a Pixel, open Settings > Network & internet > SIMs. A greyed-out carrier with "No SIM" means the card is missing or inactive; if it's simply switched off, turn on Use SIM. Then eject the tray with the SIM tool or a paperclip and look at the card. Bent or damaged means a replacement from your carrier. Dirty means a wipe with a microfibre cloth. After that, swap SIMs between phones: yours in another handset, another card in yours. Whichever combination fails tells you whether the card or the phone is at fault.

On a Samsung Galaxy the sequence is restart, power off and inspect the card, clean card and tray with a dry cloth, then reset mobile network settings under Settings > General management > Reset. Read Samsung's warning before that last step. The reset can't be undone, it wipes any custom network configuration, and Samsung says you may not be able to reconnect afterwards, depending on what your carrier requires. On a local SIM abroad, where you may have typed in settings at the counter, that matters.

SIM PIN and PUK lock: stop guessing

A SIM PIN locks the card itself, not the phone. Enter it wrong too many times and the card demands a PUK, the Personal Unlocking Key. Get the PUK wrong too and the card is finished.

Apple is blunt about it: "Never try to guess a SIM PIN or Personal Unlocking Key (PUK) code." A wrong guess can permanently lock the SIM or eSIM, and if you see "PUK exhausted" the only remedy is a new SIM from your carrier. Verizon's FAQ puts a number on it: three wrong PUK entries in a row permanently lock a Verizon SIM. It also says every Verizon SIM ships with a default PIN of 1111, and that only the account owner or manager can look up the PUK in My Verizon. Other carriers use other defaults, so don't assume 1111 elsewhere.

So the PUK comes from the carrier that issued the card, through its app, account or support line. A "free PUK code" website can't produce it, because the code belongs to that specific card. Many carriers print it on the plastic card the SIM was snapped out of, so if you bought a local SIM abroad, keep that card until you fly home.

"Unlock SIM card" usually means unlocking the phone

Lots of people searching "unlock sim card" actually have a carrier-locked phone, not a locked SIM. The symptoms overlap: a new SIM goes in and the phone refuses it.

No SIM card unlocks a phone. The lock lives in the handset, and Apple's unlocking page says only your current carrier can lift it, which may take a few days. Check Settings > General > About first; "No SIM restrictions" means you're already free. If it says anything else, request the unlock before you travel, not from a shop counter in another country.

Local SIM not working abroad

This is the case we care about most, and it has a short list of suspects.

The phone is carrier-locked. A locked phone rejects every foreign SIM, and a travel eSIM won't install on it either. It's the most common cause and the one you can't fix on the day.

Data is on the wrong line. On a dual-SIM phone, the phone may still be pushing data through your home line. On an iPhone, Settings > Cellular > Cellular Data lets you choose; on a Pixel it's Network & internet > SIMs > Mobile data.

The APN is wrong or missing. Calls work, data doesn't. Apple keeps the APN under Settings > Cellular > Cellular Data Network, warns that wrong settings can cost you data and extra charges, and says not to edit it unless your carrier tells you to. Abroad, the shop that sold you the SIM is that carrier. Ask them to check it before you walk away.

The SIM isn't registered yet. Several countries won't let a prepaid SIM work until it's tied to your passport. In Italy and Vietnam the shop registers it before it works; in the Philippines the law requires a passport and a photo at the counter; India wants passport, visa and a photograph. A SIM sold without that step may simply sit there showing no service.

Our rule: test the SIM at the counter. Make a call, load a web page, then leave. Everything above is a two-minute fix with the seller in front of you and a miserable afternoon without them.

When it's an eSIM that isn't working

eSIMs fail differently: QR codes that won't scan, plans that never activate, a line that shows signal and no data. We cover those separately in Airalo eSIM not working and Holafly eSIM not working, and if you're still choosing a phone, eSIM compatible phones lists which ones take an eSIM at all.