Every step below is from an official source
Switch from T-Mobile to AT&T
What to do at T-Mobile before you leave, what AT&T needs when you join, and everything else the switch touches. Your full plan is built in your browser and stays there.
Official sources reviewed . How we review sources
The rules that apply: Federal Communications Commission
These apply whichever mobile carrier you're moving between.
Contact your new company to start the portBefore
You start porting your number by contacting the new company, not the old one.
Portability: keeping your phone number when changing service providersU.S. Federal Communications Commission. ReviewedKeep your old service until the new one startsBefore
Don't end service with your current company before you start new service with another company.
Portability: keeping your phone number when changing service providersU.S. Federal Communications Commission. ReviewedGive the new company your number and requested detailsBefore
Give the new company your 10-digit phone number and any other information it asks for.
Portability: keeping your phone number when changing service providersU.S. Federal Communications Commission. ReviewedKnow your old company can't refuse to portDuring
Once you request service from a new company, your old company can't refuse to port your number, even if you owe money. You may still have to pay early termination fees or balances under your contract.
Portability: keeping your phone number when changing service providersU.S. Federal Communications Commission. ReviewedExpect a wireless port to take a few hoursDuring
When you switch between wireless providers, you may be able to use your phone within a few hours.
Portability: keeping your phone number when changing service providersU.S. Federal Communications Commission. Reviewed
At T-Mobile, before you leave
Generate a Temporary Port Out PIN as primary account holderBefore
Only the Primary Account Holder can generate a Temporary Port Out PIN. You can do it in the T-Life app or on T-Mobile.com.
Transfer your phone numberT-Mobile. ReviewedTurn off Port Out Protection if it's onBefore
If Port Out Protection is active on the line, disable it so the Temporary Port Out PIN works.
Transfer your phone numberT-Mobile. ReviewedGive your new provider your account number and PINsBefore
Contact your new provider to start. Have your T-Mobile account number from your account or bill, the Temporary Port Out PIN, and your account PIN or passcode.
Transfer your phone numberT-Mobile. ReviewedCheck whether your device can be unlockedBefore
Check device unlock status in your T-Mobile account. Once a device is eligible, T-Mobile unlocks it automatically within two business days if it supports remote unlock.
T-Mobile device unlockT-Mobile. Reviewed
At AT&T, when you join
Choose Keep my number when you order onlineBefore
When you order online from AT&T, choose Keep my number at checkout.
Have your account number and transfer PIN readyBefore
Have the name on your current plan, your current account number, and a number transfer PIN or account code if your carrier uses one.
Don't cancel your current wireless serviceBefore
Your number must stay active so AT&T can bring it over. Your current service keeps working until the transfer is complete.
Then: everything else the switch affects
The steps above are what T-Mobile and AT&T say. Your full checklist adds the accounts, codes, payments and devices that depend on your mobile carrier, in a safe order. It's built from your answers, in your browser.