A:AnswerI'm sorry I really can' answer this question because I paid cash for the phones. Maybe ask a Best Buy Mobile Agent next time you are in the store.
A:AnswerThat is normally carrier dependent, but I have a Verizon grandfathered unlimited data plan and I upgraded to the Pixel XL last year when it first came out and I didn't have to change my plan. Consequently, I also have an AT&T plan that I wasn't able to keep my grandfathered unlimited data plan with after upgrading my iPhone. That was a few years ago though.
A:AnswerIf you want unlocked GSM phone. Purchase it from Apple website either T-Mobile or AT&T. (T-Mobile is best, bcz you don't need to enter any existing account details) and purchase at full price. It will come as Unlocked and you can use it with any supported GSM carriers across Globe (List of carriers are included in Apple website)
This I have confirmed with Apple Customer Support.