If you're on one of U.S. Cellular's Total Plans, there's no charge to connect your Apple Watch to the Cellular network. If you're on any other U.S. Cellular plan, you'll get 3 months for a free trial and after that there is a $4.99 monthly charge to connect your Apple Watch to the Cellular network.
The PIN is the number you chose when you first activated your U.S. Cellular service. The PIN is typically used for account verification online, over the phone, or in a store. If you cannot remember your PIN, please visit your nearest retail location with a government-issued photo ID or call Customer Service at 1-888-944-9400.
Apple Watch can be used while roaming within the United States. However, international roaming is not supported.
Your iPhone and Apple Watch should both be signed in to the same Apple ID. On iPhone, go to the Settings app. You can sign in to iCloud from there. On your Apple Watch, you can sign in to your Apple ID when pairing your Apple Watch to your iPhone. Or, if you skipped that step during setup, you can sign in from the Apple Watch app on iPhone under General > Apple ID. To send and receive SMS from Apple Watch over cellular, your iPhone needs to be signed in to iMessage, turned on, and connected to a cellular network.
AppleCare+ for Apple Watch extends your coverage to two years from the original purchase date of your Apple Watch and adds up to two incidents of accidental damage coverage. In addition, you'll get 24/7 priority access to Apple experts by chat or phone. AppleCare+ can be added within 60 days of your Apple Watch purchase:
Online (requires you to verify your serial number and run a remote diagnostic)
Visit an Apple Store (inspection of Apple Watch and proof of purchase are required)
Call (800) 275-2273 (requires you to run a remote diagnostic and provide proof of purchase)