For some specific info, I bought a phone from Amazon in 2018:
Samsung Galaxy S9 Plus (6.2", Single SIM) 128GB SM-G965F (GSM Only, No CDMA) Factory Unlocked LTE Smartphone (Midnight Black) - International Version
After receiving this phone 5 years ago, I then drove to a T-Mobile store so they could set up service for me. Now 5 years later, I am changing carriers. My last payment to T-Mobile was January 18, 2023, so I assume my service expired February 18, 2023 which was three months ago. The new carrier I would like to have is Mint. I have purchased a Mint sim card and inserted it into my phone. I called a Mint technician with the Mint sim card. The tech told me my phone is locked. I later called a T-Mobile tech with another phone. They seem to not have my account anymore.
My questions then are about phone locking. If I bought an unlocked phone off of Amazon, would T-Mobile have locked the phone when I got T-Mobile service? Should Mint be able to just activate the sim card and get it to work?
I am confused about the idea of phone locking. Is this something that is done to phones that are purchased from a carrier, but not done to phones that are purchased from a manufacturer? Is this something the carrier must do to activate service, and then they must unlock when service expires?
