Telephone lists
My husbands phone list has appeared on my iPhone11 merged with mine. I have 33 numbers and he has 1150. It has my details on the “my card” and not his so can’t find a way of separating them again.
iPhone 11, iOS 16
Question marked as
Are you using the same Apple ID on both devices?
If yes, you should use your own Apple ID, to stop merging information for contacts, calendars etc.
You can use Family Sharing to share content and purchases after that.
Set up Family Sharing - Apple Support
Switch off "Find my" in the settings for the Apple ID, so that you can restore your phone to factory settings and set it up with the new Apple ID: