My carrier won’t unlock my iphone

Most carriers have requirements for requesting an unlock; typically that the device be paid off/owned by you. You can find T-Mobile details at https://www.t-mobile.com/support/devices/unlock-your-mobile-wireless-device

No. Only the carrier a phone is locked to has the ability to unlock the phone for use with a different carrier. You'll have to work this out with T-Mobile. Apple can't help you. We can't help you.

Ps: They likely are unwilling to unlock your phone because you are under contract to them, they subsidized the purchase of your phone or you owe them money.