How can i deregister a phone number i no longer have access too?

You can try the below.

iMessage on your iPhone or online - Deregister.   

You can also change/remove the number on your Apple ID. After the change, allow up to 24 hours for the change to sync. Before trying to make the change, sign out of Messages/FaceTime.

Apple ID - Manage   

Have you signed out of Messages/FaceTime on all devices/computers and allowed time for the change to sync?

Do you have 2 Step Verification or 2 Factor Authentication enabled on your account and the number you want to remove is your only trusted number? Do you have a new number to add?

Did you try accessing icloud.com and changed the number ?

I just logged in to my Apple ID but there is only a ? next to the number I want to remove and not an X. How do I resolve this?