Model number. The carrier unlocked version has a different model number than the Tmobile version.

I understand that they are both SE’s 2nd Gen but the clerk who sold them gave you the Tmobile version by mistake.

thanks @gramps28 for bringing up the model number difference. Looks like that is the key to why one phone would lock and the other wouldn’t. The flex policy that @syaoran brought up does make unlocking interesting. @jyuan2004 Since the phone was purchased through Apple, the unlock process is a little bit different. If you remove the sim card and reset the phone, it should automatically unlock during that process and be ready to use the new sim card. If you do the reset and it still says that it is not compatible with a non-T-Mobile SIM, then you would need to reach one of our care experts and most people on the Community like to do that by direct messaging us through Facebook or Twitter. You’ll tell them your situation and then ask them to follow the process for iPhones purchased from 3rd party retailer. That request takes a couple days, but it ensures that all systems know that the phone should not be locked to T-Mobile.

Are you a Tmobile customer? If not Tmobile won't be able to help and you need to contact the person you bought it from.

I am experiencing the same problem. I bought a phone from Saudi Arabia and it was replaced by Apple. Currently, no SIM card works on it in the settings. When I go to General, then it appears carrier lock ( SIM LOCKED ) I need help please 

Also how can I contact T-Mobile

I am not but there was a T-mobile sim in my phone also I am not in USA and I got my iPhone from Saudi Arabia who can help me?

Iit was woking for 2 years but now I traveled to Turkey and it doesn’t work with any sim 

Tmobile will only help Tmobile customers. Sounds like your phone was jailbroke which isn't a permanent unlock code.

It will unlock again or what I don’t get what you said 

also I spoke with Apple customer service they told me that I need to talk with T-mobile because I told them when I got my iPhone it was T-mobile sim in and I searched for email or something to talk with T-mobile but I didn’t find so now you try to say I can’t use my iPhone again with any sim  or what?

Are there any solutions for this issue?

I really need help with this issue 

Tmobile did not sell you the phone, you bought a USA iPhone on the Grey market from a seller who probably jailbroke it. Tmobile will not help since you're not a customer.

You can try taking to a phone repair store in Turkey to see if they can jailbreak it again for you but there's no guarantee it will not lock again down the road.

Ok thanks gramps28 I appreciate that 

You can thank Apple's reseller flex Policy for the device locking to the first SIM you out in it.  If you meet all of the SIM Unlock Policy requirements, you can request that T-Mobile submit to Apple to remove the SIM lock on your device.  You must be a T-Mobile USA customer and your account and device must meet all of the SIM Unlock Policy requirements.  If the device was not purchased from T-Mobile.  You will need to submit the original sales receipt along with your request. 

https://www.t-mobile.com/responsibility/consumer-info/policies/sim-unlock-policy

If each device would lock to the first SIM, why the other phone didn’t have this lock? This story smells fishy.

I imagine that the 2 lines were activated by 2 technicians, and one did the right things, and the other made a mistake. For devices that owe T-mobile money, the lock should be applied. For devices that don’t owe money, they should not be locked. What should be done in each case is up to the technician to follow the procedures. Anyway, I don’t want to blame the technician for the mistake. We are human and mistakes can happen.

My desire is to remove the lock on my phone. The procedure is a bit too complicated as of now, it seems. I hope they will solve this problem and develop a good procedure so that other technicians can follow and other customers can avoid the troubles that I got today.

Are they both the same model number? You may have received a fully unlocked phone and one that locks to the first carrier.

Both are iPhone SE 2nd gen, and bought in 1 order from Apple, fully paid, in 2020.

I have always used T-mobile in the recent 15 years. So, there is no other carrier in this picture.

The model numbers are different:

NX9M2LL/A <--with carrier lock

MX9N2LL/A <--without carrier lock

Thanks for the tip!

In April/May 2020, all Apple stores were closed due to the onset of COVID-19. So I ordered them online. They came in separate packages, but I didn’t give it much thought. I never paid any attention to the models, and it’s quite a surprise to see that they are different!

Model number makes a difference.

also by chance was the one that is unable to be unlocked ever insurance/warranty replaced?

Both phones have been working flawlessly since day one. There has never been any repair or replacement.