Switching US Carriers potential issues?

[Vaotix]

I have an AT&T version of the S21 Ultra and have been considering moving to T-mobile. Do I need to have my phone unlocked by AT&T first or will it just work on T-mobile? And if so, will it be lacking any features because it's not a t-mobile version of the phone? 

[Anonymous · Accepted solution]

If it's unlocked it will switch to t-mobile firmware with no problem once you switch the Sim to tmobile

[Vaotix]

Thanks. I'm not sure if it's unlocked or not. Guess I'll have to call AT&T to find out.

[AI4GKDan]

No. Go to the T-Mobile store. They'll take care of everything for you. They want me subscribers. It'll likely auto-unlkck. Otherwise, they can go online and do the switch. All they need is your AT&T account number and maybe your PIN.

[KiwiBird97]

That's not quite true. The KR/US/CA/JP versions (4 separate models) will work on any other carrier, but only within the same region. Here's how to check if your CSC is supported (no root needed).
1. Enable debugging features in Settings > Developer Options, and decide whether you want to use USB or Wi-Fi debugging. Allow when prompted.
2. From a terminal, input "adb shell" (Mac users add a ./ at the beginning).
3. Input "cd prism/etc/carriers" or "cd optics/configs/carriers" (after running, you may need to enter "cd single" to show all the CSCs).
4. Input "ls" to see all CSC codes.
Please ignore the quotes in all terminal inputs.

On a Tab S8 Ultra Wi-Fi, here's what we got.
• BRI: Taiwan
• CAU: Caucasus
• EUX: Europe X
• EUY: Europe Y
• ILO: Israel
• INU: India
• KOO: Korea
• MXO: Mexico
• SER: Russia
• TGY: Hong Kong
• TUR: Turkey
• XAC: Canada
• XAR: United States
• XSG: Middle East
• XSP: Asia Pacific
• ZBR: Latin America
(WWA to WWD are skipped since they are all retail demo CSCs.)

If we look at the unlocked S22 Ultra CSC list for the USA (SM-S908U1), we saw a completely different list, which surprised us a lot. No CSCs are from OUTSIDE the USA.
• ATT: AT&T
• AIO: Cricket
• CCT: Xfinity Wireless (Comcast)
• CHA: Spectrum Mobile (Charter)
• DSH: Dish
• TFN: Tracfone
• TMB: T-Mobile
• USC: US Cellular
• VZW: Verizon
• WWD: Retail Demo
• XAA/XAR: United States (unbranded)
• And other MVNOs, as far as I know

To be honest, unlocked (not Wi-Fi only) Galaxy devices sold in Korea, USA, Canada, and Japan do not work properly when it comes to studying abroad, in terms of the SIM card and language restrictions. Samsung should at least use the Wi-Fi variant's CSC template instead when it comes to unlocked variants.

To recap, assuming you've unlocked the phone and changed the SIM:
• If the SIM card's carrier is in the SAME region and is in the CSC list, the new carrier settings and apps will be added, including any APNs. No further action is needed.
• If it is in a DIFFERENT region, but in the CSC list, the phone will tell you to switch to your home region's SIM and make phone calls for at least 5 minutes to remove the restriction. (Europe, Latin America, Middle East, and Asia Pacific are considered larger regions. Under normal circumstances, this should have already been completed after making your first few voice calls.)
• If it is NOT in the CSC list, the SIM card will be rejected and the phone will tell that the SIM card isn't supported. Your phone will not have any cellular service since no APNs are defined due to the fact that it is not in the CSC list. For example, you can't use a Rogers SIM card on a USA S22 Ultra.
(The sentence "Your phone can be used with any compatible service provider" appears to be vague, unless the phone was NOT purchased in KR/US/CA/JP. We don't want to waste money for a duplicate!)

[Anonymous]

Just try a sim card from t-mobile and see if it accepts Sim card

[Vaotix]

I don't have one and haven't made the switch yet.

[Anonymous]

👍

[ChazzP12]

If your phone is at&t carrier locked then Your phone will NOT work on t-mobile... the only way your phone to work on any other carrier is to have at&t to UNLOCK your phone... once unlocked you can use on any carrier...

[usermzeJaY3BsM]

Likely AT&T will require you to fully pay off your device before they will unlock it, in case you have not already done this.