You don’t need to activate it with T-Mobile as far as I know. You do however, have to make sure to request an unlock from T-Mobile since your phone is fully paid off. I’m not sure if T-Mobile iPhones are unlocked like the Verizon ones. I have Verizon. I’m only in this thread because I bought my girlfriend a phone for her account.
When they agree to unlock your phone, you will get the following email within 48 hrs:
"Unlock Code: SEE BELOW
Thank you for taking the time to contact T-Mobile. 24 hours after receiving this email, follow the below instructions for unlocking your T-Mobile iPhone.
If you have a non-T-Mobile SIM card, follow these steps:
If you have a non-T-Mobile SIM card:
1. Insert non T-Mobile SIM Card
2. Complete the setup process.
If you will be using your T-Mobile SIM card follow the steps below:
1. Back up your iPhone using iTunes.
2. When you have a backup, erase your iPhone.
3. When you see the Welcome screen, connect your device to iTunes.
If successfully unlocked, the message Congratulations, your iPhone has been unlocked will display. Additional information for unlocking Apple iPhone devices can be located at
https://support.apple.com/en-us/HT201328
Thank You,
Customer Service
T-Mobile USA, Inc."