Just got iPhone 6s from a friend and my T-Mobile SIM card won't work.

How do you know it is the right Sim for the iPhone 6? Take the phone and the Sim card into a T-Mobile store and let them figure it out. Only T-Mobile can unlock the phone for their service. We can't help with that.

Good luck.

The iPhone is locked to a specific carrier. You need to get a SIM from that carrier.

Simsupport843 Said:

"when I put my sim in my iPhone 6s it’s says insert a sim supported by carrier which it’s a T-Mobile sim and T-Mobile phone what do I do"

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Get the correct SIM card from your carrier.