Deregister phone number
Phone I don’t have deregister
Question marked as
There's not a lot to go on with your question. I'm going to assume that you have your old number still listed on your Apple ID?
Go to Settings -> click on your name at the top -> Name, Phone Numbers, Email (it may ask for your Apple ID password)
Under the "Reachable at" section, click on Edit. you can then click the circle next to your number and click on deregister.