Getting Your AT&T SIM Card Ready to Use

What you need to do to start using your AT&T SIM card

Activating an AT&T SIM card means connecting it to your AT&T account so your phone can make calls, send texts, and use data. The process depends on whether you're using a new phone, switching from another carrier, or replacing a damaged SIM. Most activations happen online through AT&T's website or the AT&T mobile app, though you can also call AT&T directly or visit a store. The whole process usually takes a few minutes to a few hours.

You'll need your SIM card number (printed on the card itself), your phone's IMEI number (found in Settings or on the phone's box), and your AT&T account information or the phone number you want to use. If you're switching from another carrier, have your account number from that carrier ready — AT&T will need it to port your number over.

Key Takeaways

You can activate your AT&T SIM card online through AT&T.com, through the AT&T mobile app, by calling 611 from an AT&T phone or 1-800-331-0500 from any phone, or in person at an AT&T store.
Online activation requires your SIM card number, your phone's IMEI number, and either your AT&T account details or the phone number you want to use.
If you're moving your number from another carrier, AT&T will port it during activation, which usually takes 24 hours but can take up to a few business days.
Your SIM card will not work until activation is complete, even if you insert it into your phone and see signal bars.
If activation fails online, calling AT&T customer service is faster than visiting a store, since they can troubleshoot immediately.

Activating through AT&T's website or app

Go to AT&T.com and sign into your account, or download the AT&T mobile app and sign in there. Look for the option labeled "Activate a device" or "Add a line" — the exact wording varies depending on whether you're adding a new line or replacing an existing SIM. You'll be asked to enter your SIM card number, which is printed on the physical SIM card itself and also on the packaging it came in.

Next, enter your phone's IMEI number. You can find this by opening Settings on your phone, going to About Phone, and looking for IMEI or IMEI 1. If your phone is brand new and won't turn on, the IMEI is printed on the box. AT&T will use this number to confirm your phone is compatible with their network.

Choose whether you're keeping your existing AT&T number, using a new number, or porting a number from another carrier. If you're porting, enter the account number from your old carrier — you can usually find this on your old bill or by calling that carrier's customer service. AT&T will then confirm the details and complete the activation. You should see a confirmation message within a few minutes.

Activating by phone or in a store

Call 611 from any AT&T phone, or 1-800-331-0500 from any phone. An AT&T representative will ask for your SIM card number, phone IMEI, and account information. This route is useful if you're not comfortable entering the information online or if you run into an error during web activation. The call usually takes 10 to 15 minutes.

You can also visit an AT&T store in person. Bring your SIM card, your phone, and a photo ID. A store associate will activate the SIM for you on the spot. This is the slowest option if the store is busy, but it's helpful if you need immediate help troubleshooting a phone that won't connect after activation.

What happens after you hit activate

Once AT&T processes your activation, insert the SIM card into your phone (or make sure it's already inserted), power the phone off and back on, and wait a few minutes. Your phone should connect to the AT&T network and you'll see signal bars appear. You can then make a test call or send a text to confirm everything is working.

If you're porting your number from another carrier, that number will move to AT&T during activation. The transfer usually completes within 24 hours, though it can take up to a few business days. During this time, your old carrier's service will stop working. You'll receive a text message from AT&T confirming when the port is complete.

If your phone doesn't show signal bars after 30 minutes, restart it again. If it still doesn't connect, call AT&T at 1-800-331-0500 to confirm the activation went through — sometimes the system processes the activation but your phone needs a manual network reset to recognize it.

Troubleshooting activation problems

The most common issue is entering the SIM card number incorrectly. Double-check that you're copying it exactly as printed — it's usually a 19 or 20-digit number on the back of the physical card. If you're using a SIM that came in packaging, verify the number on both the card and the box match.

If AT&T says your phone's IMEI is not compatible, your phone may not support AT&T's network bands. This happens most often with phones bought outside the US or phones from carriers that use different technology. Call AT&T customer service to confirm — they can sometimes override this, or they can tell you whether your phone will work at all.

If activation completes but your phone still has no signal after restarting, the SIM card itself may be damaged or defective. Request a replacement SIM from AT&T — they can mail one to you or you can pick one up at a store. Activation of the replacement SIM is free.

Porting your number from another carrier

During activation, AT&T will ask whether you want to keep your existing number or get a new one. If you want to keep the number you have with another carrier, select "Port my number" and enter your old carrier's account number. AT&T will contact your old carrier to request the transfer.

The port usually completes within 24 hours. Once it's done, your old carrier's service stops and all calls and texts go to your AT&T number. You'll get a text from AT&T confirming the port is complete. If the port takes longer than a few business days, call AT&T to check the status — delays sometimes happen if your old carrier has trouble locating your account.

Important: Do not cancel your old carrier's account until the port is complete and you've confirmed your number works on AT&T. If you cancel early, the port may fail and you could lose the number.

When to contact AT&T support

Contact AT&T if activation fails after you've entered all your information correctly, if your phone shows no signal 30 minutes after activation, if you're not sure whether your phone is compatible with AT&T's network, or if a ported number hasn't arrived after three business days. Call 1-800-331-0500 from any phone — wait times are usually shortest in the early morning or late evening.

Have your SIM card number, phone IMEI, and account number ready when you call. If you're porting a number, also have your old carrier's account number. AT&T support can usually resolve activation issues in one call.

Frequently Asked Questions

Can I activate my SIM card before I get my phone?

No. AT&T needs your phone's IMEI number to complete activation, and you can't get that until you have the phone. You can order the SIM card ahead of time, but activation has to wait until the phone arrives and you can look up its IMEI.

Do I have to go to a store to activate, or can I do it online?

You can do it entirely online through AT&T.com or the AT&T app. Going to a store is optional and usually slower. Online activation takes a few minutes and works for most people.

What if I activate the SIM but my phone still says no service?

Restart your phone and wait a few minutes for it to reconnect to the network. If that doesn't work, call AT&T at 1-800-331-0500 to confirm activation completed. If it did, ask them to send a network reset command to your phone, or request a replacement SIM card.

How long does it take to activate a SIM card?

Online activation usually completes in a few minutes. If you call AT&T, it takes 10 to 15 minutes. If you're porting a number from another carrier, the number transfer itself takes 24 hours but can take up to a few business days.

Can I activate a used or secondhand SIM card?

Yes, as long as it's an AT&T SIM card and it hasn't been reported as lost or stolen. AT&T will deactivate any previous account tied to it and set it up for your account instead.

This guide is for general information only and is not medical, financial, legal, or other professional advice. For decisions specific to your situation, consult a qualified professional. See our Editorial Policy.