A:AnswerHi. The Apple Watch works with most carriers. It just happens that you were viewing a model that was for AT&T. They also show models for Verizon on the Best Buy website. I do not see models for Sprint. However, below I have added the links to two Apple Watch Nike edition that are Unlocked versions and are for cellular service. I assume you would just have to call sprint to have it activated.
A:Answeryes, this works with Sprint and all of the carriers... the watch has an e-sim that works with them all. at the time of my purchase, the first 3 months were free with Sprint, but after that it will be $5 extra a month.
A:AnswerYou do not need ATT, but not all providers are able to give you the SIM card yet, I am with US Cellular and I can't get a SIM card until next year(2018), first quarter, but I can still use the watch pretty far away from my phone. It's soon enough I wanted the phone!