A SIM card is the small chip that connects your phone to your carrier's network and gives you a phone number. Put it in, and it usually just works. When it doesn't, or when you travel, change phones or get asked for a PUK code, it helps to know your way around your SIM card settings. This guide is the practical part: where the SIM settings are on iPhone and Android, how to lock your SIM with a PIN, SIM card tips, quick fixes and how to use your SIM abroad. For the basics, see what a SIM card is and the different types of SIM cards.
SIM card basics in one minute
- What it does: your SIM identifies you to your carrier, so you can make calls, send texts and use mobile data on your plan.
- What it stores: the identity numbers and keys your carrier uses to recognise you, like the IMSI and the ICCID. Some phones can also save contacts to the SIM.
- Your number lives with your carrier: the SIM links your phone number to your account. Here's how SIM cards, carriers and phone numbers are related.
- Sizes: most phones today take a nano SIM. Some have no SIM tray at all and use an eSIM, a digital SIM built into the phone.
How to insert and activate a SIM card
Insert a SIM card
- Turn off your phone.
- Find the SIM tray on the side of the phone. Push the SIM tool (or a straightened paper clip) into the small hole to pop it out.
- Place the SIM in the tray. The cut corner only fits one way, and the gold contacts usually face down.
- Slide the tray back in until it's flush, then turn your phone on.
Older phones may have the slot under the back cover. Don't force a SIM into the wrong size slot. If your SIM is too big, ask your carrier for one in the right size.
Activate a SIM card
Most SIM cards activate when you put them in and turn the phone on. Some carriers ask you to activate the SIM online, in their app or by text first. In many countries you also need to register the SIM with an ID before it works. If it still doesn't connect after a restart, check the instructions that came with the SIM or contact your carrier.
An eSIM is set up in your phone's settings, usually by scanning a QR code or through your carrier's app.
SIM card settings on iPhone and Android
All your SIM settings sit in one place. Menu names vary a little by phone model and software version.
- iPhone: Settings > Cellular (called Mobile Data in some countries).
- Google Pixel: Settings > Network & internet > SIMs.
- Samsung Galaxy: Settings > Connections > SIM manager, with mobile data and roaming under Connections > Mobile networks.
Turn a SIM on or off
On an iPhone, go to Settings > Cellular, tap the line and switch it on or off. On a Pixel, go to Settings > Network & internet > SIMs, tap the SIM and turn off Use this SIM (Google: Use dual SIMs on Pixel). This is what "enable SIM card" means: the line is switched on and can connect.
Choose a line for calls, texts and data
With two SIMs (or a SIM and an eSIM), you pick a default line for calls and texts and one line for mobile data. Only one line can use mobile data at a time. On iPhone, go to Settings > Cellular > Cellular Data and tap the number to use. Give each line a label, like "Home" and "Travel" (Apple: Use Dual SIM with an eSIM).
Mobile data and data roaming
Mobile data is your internet connection over the cellular network. Data roaming lets that connection use other networks when you're outside your carrier's coverage, usually abroad. Both are set per line.
- iPhone: Settings > Cellular > Cellular Data Options > Data Roaming. With two lines, tap the number first (Apple: Data roaming options).
- Pixel: Settings > Network & internet > SIMs, then turn Roaming on or off (Google: Manage mobile data usage).
APN settings
The APN (Access Point Name) tells your phone how to connect to your carrier's data network. Most phones set it up on their own. If mobile data doesn't work, your carrier may ask you to check it.
- iPhone: Settings > Cellular > Cellular Data Network, if your carrier allows changes. To undo your changes, tap Reset Settings at the bottom. Apple advises not to edit the APN unless your carrier tells you to (Apple: View and edit your APN).
- Pixel: Settings > Network & internet > SIMs, tap the SIM, then Access point names (Google: Connect to mobile networks). Other Android phones have it under the mobile network settings.
More detail: what an APN is and how to change it.
Carrier settings update (iPhone)
Carriers sometimes send updated network settings. Go to Settings > General > About. If an update is available, you'll see a prompt to install it.
Reset network settings
If calls or data keep failing, resetting network settings is a good last step before calling your carrier.
- iPhone: Settings > General > Transfer or Reset iPhone > Reset > Reset Network Settings. This removes saved Wi-Fi networks, VPN settings and other network settings, and may turn off data roaming. Your photos, apps and data stay. Deleting eSIMs is a separate option in the same menu, so don't tap Delete All eSIMs by mistake (Apple: Reset iPhone settings).
- Pixel: Settings > System > Reset options > Reset Mobile Network Settings.
SIM PIN and PUK: lock your SIM safely
A SIM PIN stops anyone from using your SIM in another phone, or in yours after a restart. It works on a physical SIM and on an eSIM.
Turn on your SIM PIN
- iPhone: Settings > Cellular > SIM PIN. With two lines, tap the line first. After that, your SIM locks every time you restart the phone or remove the SIM, and you'll see "Locked SIM" until you enter the PIN (Apple: Use a SIM PIN).
- Android: search for "SIM lock" in the Settings app. It's usually under the security settings, as SIM card lock or SIM lock.
You'll need your current PIN to turn it on. If you never set one, your carrier uses a default PIN, like 1111 on EE in the UK. Check your carrier's site or the card your SIM came on, and change the default to your own PIN.
If your phone asks for a PUK code
Enter the wrong SIM PIN three times and the SIM gets blocked. To unblock it, you need the PUK code from your carrier. Enter the wrong PUK ten times and the SIM is blocked for good, so you'll need a new SIM (EE: Turn SIM PIN on or off). Apple's advice is simple: don't guess. Ask your carrier for the PUK instead.
Where to find it: what a PUK code is and how to get it.
SIM card tips
- Write down your PUK before you need it, and keep it away from your phone.
- Set your own SIM PIN instead of the carrier default.
- Protect yourself from SIM swap fraud. Ask your carrier for an account PIN or a lock on number transfers, and use an authenticator app instead of text codes where you can. More on SIM swapping.
- Label your lines if you use dual SIM, so you always know which one pays for calls and data.
- Keep a prepaid SIM active. Many carriers close prepaid numbers that go unused for a while. Check your carrier's rules.
- Check your balance with a short code. Many carriers let you check credit or data with a USSD code like *100#.
- Know your phone's IDs. Dial *#06# to see your IMEI and, on eSIM phones, the EID. You'll need them if your phone is lost or you switch carriers.
- Handle a physical SIM with care. Hold it by the edges and keep the gold contacts clean and dry.
Switching SIM cards and phones
Put your SIM card in another phone
Your SIM works in another phone if it fits the tray and the phone isn't locked to a different carrier. Your number, plan and carrier move with the SIM. Your photos and apps don't, since they're stored on the phone or in the cloud. More in can I put my SIM card in another phone.
Replace a SIM card and keep your number
Changing your SIM card doesn't mean changing your number. If your SIM is lost, damaged or the wrong size, your carrier can give you a replacement SIM or an eSIM with the same number. If you move to a new carrier, ask to transfer (port) your number.
Switch from a SIM card to an eSIM
Many carriers can move your line to an eSIM, either in your phone's settings or in their app. Some recent phones, like iPhone models sold in the US since the iPhone 14 and most Pixel 10 models in the US, have no SIM tray at all. Here's how to switch from a SIM card to an eSIM.
Lost or stolen phone
You track the phone, not the SIM. Use Find My on iPhone (Apple: Use Find My) or Find Hub on Android (Google: Find, secure, or erase a lost Android device). Then call your carrier to block the SIM and get a replacement with your number.
SIM card problems and quick fixes
"No SIM" or "Invalid SIM"
- Restart your phone.
- Check for a carrier settings update (iPhone: Settings > General > About).
- Turn the phone off, take the SIM out, check it for damage and put it back. Make sure the tray closes fully.
- Try your SIM in another phone, or another SIM in yours. That tells you if the SIM or the phone is the problem.
- Check that your plan is active and your phone isn't locked to another carrier. On iPhone, Settings > General > About shows "No SIM restrictions" next to Carrier Lock if it's unlocked (Apple: If you see Invalid SIM or No SIM).
Using an eSIM? Contact your carrier instead of reseating anything. Full guide: what Invalid SIM means and how to fix it.
Signal bars but no mobile data
- Check that mobile data is on for the right line.
- Abroad, check that data roaming is on for the line you use for data.
- Check the APN with your carrier, then reset network settings if nothing else helps.
"SOS only" or no service
Your phone can see a network but can't register on it. This often happens abroad when roaming is off or your plan doesn't cover the country. Check that roaming is on, restart the phone, and contact your carrier if it stays that way.
Using your SIM card abroad
Your SIM card usually works in another country if your plan includes roaming and roaming is turned on. Roaming can be included in your plan, sold as a daily or weekly pass, or charged per use, so check your carrier's rates before you go. This applies to prepaid plans too: many don't include roaming at all. Your options:
- Roam with your home SIM: easiest, but it can be expensive.
- Buy a local SIM card: local rates, but you have to find a shop, maybe register with your passport, and swap SIMs.
- Add a travel eSIM: install it before you fly and keep your home SIM in the phone. You need a phone that supports eSIM and isn't carrier locked. Dial *#06#: if you see an EID, your phone supports eSIM. More in our guide to checking eSIM compatibility.
Set up your phone with two lines
- Install the travel eSIM on Wi-Fi before you leave and label it "Travel".
- When you land, make the travel eSIM your mobile data line and turn on data roaming for it.
- Turn off data roaming on your home SIM, so it can't use data abroad.
- Keep your home line on if you want texts and security codes. Calls and texts on it can still cost roaming fees, so check your carrier's rates. See the correct roaming settings for an eSIM.
Airplane mode with Wi-Fi turned on is another way to avoid roaming on your home line. On iPhone, also know what Wi-Fi Assist does, since it can use mobile data when Wi-Fi is weak.
If you're heading to Europe, the USA or Africa, a data-only travel eSIM keeps you online for maps, messages and app calls, while your home SIM stays in your phone for texts and calls.
Data-only eSIMs · Europe, the United States and Africa · Europe eSIMs · USA eSIMs · Africa eSIMs · Going somewhere else? Find your destination
Need a phone number in Europe too, for example to call hotels or get calls from tour guides? Travel Basic+ from Bouygues Telecom comes with a French +33 number, calls and SMS within Europe and 20 GB of data for 15 days.
FAQs
Where are the SIM card settings on my phone?
On iPhone, go to Settings > Cellular (or Mobile Data). On a Pixel, go to Settings > Network & internet > SIMs. On a Samsung Galaxy, go to Settings > Connections > SIM manager.
What does "enable SIM card" mean?
It means the SIM or eSIM line is switched on, so your phone can use it for calls, texts and data. With two lines, you can switch one off without removing it.
What is SIM configuration?
It's the set of settings your phone needs to use your carrier's network, like the APN and your carrier's network settings. Your phone and carrier usually set this up automatically when you insert or activate a SIM.
What is the default SIM PIN?
It depends on your carrier. Some use 1111 or 0000. Check your carrier's site or the card your SIM came on. Don't guess: three wrong tries block the SIM.
Does an eSIM have a PIN and PUK?
Yes, an eSIM can have a SIM PIN. If it gets locked, ask your carrier for the PUK, just like with a physical SIM.
Will I lose my number if I change my SIM card?
No, as long as your carrier issues the new SIM for your line, or you transfer your number to a new carrier.
Will my SIM card work in another country?
Usually, if your plan includes roaming and data roaming is turned on. Check the rates with your carrier first, or add a travel eSIM for data.
Can a SIM card be tracked?
Your carrier knows which network cells your SIM connects to. To find a lost phone yourself, use Find My on iPhone or Find Hub on Android. They track the phone, not the SIM.