On AT&T's website, for example, the company says Apple Watch Series 3 connectivity requires the NumberSync feature. NumberSync, which allows one phone number to be used for multiple devices, requires a postpaid wireless account.
Apple Watch Series 3 models on Verizon use the NumberShare feature to allow Apple Watch users to accept calls on either device. NumberShare requires a "standard monthly plan" to work. On T-Mobile, the Apple Watch uses the DIGITS number sharing feature, not available to prepaid customers.
Sprint's website only lists an "active handset" as a requirement, but Fortune this week received word that all four carriers will require a standard monthly plan, including Sprint. Regular monthly plans will be needed for all customers who want an LTE Apple Watch.
Because the Apple Watch Series 3 does not get its own phone number, there's a second requirement on all carriers -- a cellular plan that already includes an iPhone 6 or later.
Sprint, AT&T, T-Mobile, and Verizon all plan to charge customers $10 per month to add an LTE Apple Watch to their accounts. Three of the four, AT&T, T-Mobile, and Verizon will be waiving their activation fees, and all are offering three free months of connectivity either up front or through service credits.
The LTE Apple Watch became available for pre-order last night, and the first orders will begin arriving to customers on Friday, September 22, the official launch date for the device. Pricing on the LTE Apple Watch Series 3 starts at $399.