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A new phone will require it be paired to the car anew. It has its own Bluetooth signature so even if restored from the backup of a previous iPhone it needs to be paired to the car. Check that you have the Tesla app on the phone and are logged into your Tesla account in the app. Go to the car with your keycard and follow the instructions to pair the phone as a key.
Once the phone is paired as a key you will be able to associate it with an existing profile in the car if you wish.
In the car change the profile to YOU and then click the "Person" icon to pair the phone as a key. When you press the "person" icon it just pairs that phone with the current profile - I expect some folk are expecting it to ask "Which profile" ... but because it doesn't it is easy to press that Icon and not realise the change is "immediate"