If you have a device payment plan with another carrier, that phone must be turned into T-Mobile to satisfy that portion of the Carrier Freedom promotion.
Hey cursedfangs! I'm so sorry to hear about your stolen phone, that's awful. snn_555 is correct here - one of the requirements to participate in Carrier Freedom is that you would trade in the phone you were making payments on in order to be reimbursed for your payment plan's balance once you upload your final bill. Do you have insurance with your current carrier? How much of a balance is left on that phone? It may be worth paying the deductible so that you can get a replacement, and then porting over, buying a T-Mobile phone and trading in the replaced phone, so that you can participate. Or, honestly, if the balance you still owe on the phone is less than a standard insurance deductible, maybe this is the right time to cut your losses and switch? That's entirely up to you, but we'll be happy to have you if you decide to do so. I'm glad that you asked here first, and I'm sorry again about your phone.