iwatch and iphone different carriers.

Hi

There are two different versions of Apple Watch (not iwatch) Series 3.

Apple Watch Series 3 (GPS) does not have a built-in cellular capability.

Series 3 (GPS + Cellular) models support a built-in cellular feature, subject to having a compatible service plan from a supported carrier, which must be the same carrier as for the paired iPhone:

• Apple Watch - Compare - Apple
• Apple Watch Series 3 - Apple

Apple Watch Series 3 (GPS)

Apple Watch Series 3 (GPS) models do not have a built-in cellular capability.

Any calls or data-related services are routed either via the paired iPhone or via a direct connection between the watch and a compatible Wi-Fi network (when the watch cannot connect to the paired iPhone via Bluetooth).

A separate cellular plan for the Apple Watch is not required.

Apple Watch Series 3 (GPS) can be used in conjunction with an iPhone 5s or newer model of iPhone (running the latest version of iOS) and a service plan for the iPhone from any carrier.

Apple Watch Series 3 (GPS + Cellular)

In respect of data usage, Apple Watch Series 3 (GPS + Cellular) also always first tries to use the Bluetooth connection to your iPhone (which does not require a separate plan for the watch) or, in the absence of that, any available connection to a compatible Wi-Fi network.

If neither of those is available, then Apple Watch Series 3 (GPS + Cellular) has a built-in cellular capability that, under certain circumstances, can enable certain features, which are not available on GPS-only models, to remain available when the watch is used without the iPhone nearby.

In order to be able to use the built-in cellular feature, you would require a carrier service plan for your iPhone that specifically includes support for the Apple Watch, including the carrier activating the watch on the plan:

• Set up and use cellular on Apple Watch Series 3 (GPS + Cellular) - Apple Support
• Use Apple Watch with a cellular network—Apple Watch Series 3 (GPS + Cellular) only

Apple Watch Series 3 (GPS + Cellular) is not locked to any specific carrier and has a built-in eSIM that copies information from the iPhone's SIM card when the cellular feature is activated.

Apple's list of supported carriers is linked below:

Apple Watch - Carriers - Apple

Apple Watch Series 3 (GPS + Cellular) can also be used without enabling the built-in cellular feature, in which case it functions like a GPS-only model and can be used in conjunction with a service plan for your iPhone from any carrier, with no need to activate the watch on the plan.

Both

The functions that are available on your Apple Watch when it is away from your iPhone depend on various factors including:

• Your Apple Watch model - Series 3 (GPS) vs Series 3 (GPS + Cellular):

  Apple Watch - Compare - Apple

• The features of your carrier service plan:
  • Apple Watch - Carriers - Apple
  • Make a call with Wi-Fi Calling - Apple Support
• The type of connectivity that is available to your Apple Watch at the time:

  About Bluetooth, Wi-Fi, and cellular on your Apple Watch - Apple Support

Features and requirements that apply under various circumstances are detailed here, including within the footnotes:

How to use your Apple Watch without your iPhone nearby - Apple Support