One thing worth mentioning here is that your iPhone and Apple Watch Series 3 (GPS+Cellular) must use the same carrier. If you switch carrier on your paired iPhone, you will have to remove the previous service plan on your Apple Watch and sign up for a new plan.
How to Change or Remove Cellular Plan on Apple Watch Series 3
How to Switch Carriers on Apple Watch Series 3
Already changed carrier on your iPhone and wish to get it done on your Watch as well? You can easily switch onto the new one.
Step #1. First off, launch Watch app on your paired iPhone.
Step #2. Now, tap on My Watch tab and select Cellular.
Step #3. Next, your carrier plan will switch automatically.
If you require adding a new plan, tap on Add a New Plan then follow the onscreen instruction.
That's it! Your watch will now quickly jump over to the new plan.
How to Remove Cellular Plan on Apple Watch Series 3
Step #1. Launch Watch app on your iPhone> My Watch tab
Step #2. Tap on Cellular.
Step #3. Tap “i” Tap on i Icon in Apple Watch App on iPhone Xbutton that's next to your carrier at the top of the screen.
Step #4. Now, you have to tap Remove “carrier” Plan. Tap again to confirm.
If you don't want to use this plan again, you need to contact your carrier to cancel your cellular subscription.
Note: While erasing and unpairing your Apple Watch, you will have the option to remove the plan from your Apple Watch. When asked, simply select Erase All to remove the plan.
Wrapping up
Apple Watch Series 3 may not boast an all-new design, but it's got much better than its predecessors. What's the one thing you like most in the new smartwatch? Let me know your views in the comments below
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Thank You
With Love
Aravind