Apple Watch Cellular without sim in iPhone

In order for cellular to work on the watch it must have a paired iPhone with an active cellular package that has an active Apple watch add on package.

If you remove the sim from the iPhone and put it in your other phone the iPhone will no longer have an active plan.

If you have an iPhone yourself you can get the Watch add on from your provider and set this watch up for a family member. That means your iPhone will work the watch. They will be able to get calls, texts on the watch.

See Set up Apple Watch for a family member - Apple Support (CA)

Apple Watch Family Plan

https://support.apple.com/en-us/109036

https://support.apple.com/guide/watch/set-up-a-family-members-apple-watch-apd54d0a51fb/watchos

Some limitations: Cannot receive SMS/MMS messages, as that depends on a fully paired iPhone. Messaging is pretty much limited to Apple’s Messages, unless the messaging service provides an Apple Watch app. Alarms, alerts, notifications on the iPhone are not sent to the Apple Watch. Apple Pay via a credit card is not available. Also not available: Medications, respiratory rate, irregular heart rhythm notifications, ECG, AFib History, Cycle Tracking, Sleep, Wrist Temperature, Blood Oxygen, Walking Steadiness, Audiobooks, Camera Remote, News, Shortcuts, and the double tap gesture.

The quick answer is no it will not work.

For the cellular to work the watch must be paired to an active iPhone that has cellular service and that there is a paid addition for the watch.

The cellular deal is the Apple Watch uses the iPhone’s number for a small additional fee to the iPhone’s cellular plan. The fee is typically $10 U.S.

If cellular is not really needed, then a GPS only Apple Watch can be used and pared to an Apple Watch that supports iOS 18. The Apple Watch will be able to do most things, including monitoring her heart, and syncing the data with the iPhone Health app. It will just not be able to make phone calls, nor access the internet outside the WiFi at home. The iOS 18 compatible iPhone must stay turned on and charged, but it can sit on a charger at home.

iOS 18 capable is non-negotiable, so the iPhone cannot be too old.

The Family Plan has limitations:

Some limitations: Cannot receive SMS/MMS messages, as that depends on a fully paired iPhone. Messaging is pretty much limited to Apple’s Messages, unless the messaging service provides an Apple Watch app. Alarms, alerts, notifications on the iPhone are not sent to the Apple Watch. Apple Pay via a credit card is not available. Also not available: Medications, respiratory rate, irregular heart rhythm notifications, ECG, AFib History, Cycle Tracking, Sleep, Wrist Temperature, Blood Oxygen, Walking Steadiness, Audiobooks, Camera Remote, News, Shortcuts, and the double tap gesture.