All Apple Watch contracts are subject to a 24-month minimum term, and to a credit check. Apple Watch plans are only available to customers on a Consumer Pay Monthly or SIM only phone plan, with an iPhone 6s or later release (running iOS 11 or later).
As your Apple Watch shares the data, minutes and texts of your phone plan, if you cancel your iPhone plan you will no longer be able to use data or receive/make calls and texts on your Apple Watch. If you cancel your Apple Watch plan within the minimum period a cancellation charge will apply.
Additional costs may be incurred when use exceeds/falls outside of your inclusive data, minutes and texts.