Remove phone number from old phone but not new phone

I got a new iPhone 12 mini today from my carrier and once I transferred my data from the iCloud Backup my phone number was already on the new phone and it was good to go. But the phone number is also still on my old iPhone 6s, and despite no longer being on my data plan and having the SIM card taken out it still receives the same calls at the same time as my new phone. So if anyone has a way to remove the phone number from the old phone but still keep the same number on the new phone that would be great.

Also, if possible I would like to be able to keep my old phone functional and connected to iCloud so I can use it if I ever need a secondary device to use on wifi if my primary device is being used for something else.