Good question and I've been thinking about it too. I
think the sequence will be:
- get February bill
- call AT&T and inform them that you want out of your contract, referencing no-ETF offer
- ask AT&T to unlock your device (maybe it can be done all in the same phone call)
- set up service with TMobile and ask the them to port the number
I don't think AT&T prorates your charges for the final month, so you might end up eating that (maybe you can be savvy and time it right to get the most out of your last month before they generate your next bill).
If you have Tmobile ship you a sim card, obviously there is some time lost there... don't know how that works if your service with AT&T has ended. I'm hoping I can stop by a Tmobile store and get a sim card from them immediately. Can you tell I haven't switched phone companies for an unhealthy period of time?
ETA: I still have my old iPhone 5s, which is unlocked. What I *might* do is set up service with Tmobile right as I'm cancelling with AT&T. If there's a bit of a delay (like getting things sorted with AT&T before they'll agree to unlock my 6s), I can just have the Tmobile service going with my old 5s. Then when everyone's done with AT&T and the 6s is properly unlocked, I'll pop the Tmobile sim in there. But I guess if I did it this way, I'd lose my phone number, so maybe not. If I had to be without cell service for a couple days, I could manage.