do i need a new phone plan for apple watch series 7 ?

If their existing carrier supports the Apple Watch, the carrier typically adds a monthly fee of around $10 U.S. to the iPhone’s bill, and the Apple Watch shares the iPhone’s phone number.

If their carrier does not support the Apple Watch, they would have to either use the Apple Watch as a GPS ONLY, get a less expensive GPS ONLY Apple Watch, or switch to a carrier that supports the Apple Watch.

Note: The GPS ONLY Apple Watch does everything the cellular model does, as long as the ipis nearby.

There is also the Family Sharing plan for dependent children, where the child does not have an iPhone, so the parent’s iPhone is used to configure and control the child’s Apple Watch. The cellular plan for the dependent child is mor expensive as the child is getting a separate phone number, but less expensive than a typical major carrier individual cellular plan.

The dependent child’s Apple Watch is not as full featured as an Apple Watch paired to an iPhone.

If you had a previous Apple Watch which was cellular, you can transfer the plan you had for it to your new watch. If you've never had a cellular Apple Watch, the link which brought you here explains how to set up a cellular plan with your carrier. This all assues your carrier supports Apple Watch. Do they?

If you plan to get them a cellular watch, you'll need to make sure the carrier they use, supports Apple Watch. If they don't, getting a cellular watch would be a wasted expense as it would operate as a GPS only watch anyway. And if they have a GPS only watch, their iPhone will have to be with them in order to receive messages, phone calls and email messages. A cellular watch won't require the associated cell phone to be on their person at all times.

Assuming their carrier supports Apple Watch, you just follow the directions in the Related Article link, which brought you here, to set up cellular on the watch. It could require you to contact the carrier at the time of setup.