Deregistering phone number

I have been trying for days now to deregister the number that came with my iphone 11 & enable my old number but it seems to be impossible. i even spent 25 minutes on the phone with an apple advisor who led me through some steps which i had already tried several times.

My problem is when I do what apple suggests which is turn off messenger & facetime go to settings, my emails etc., it shows the phone number I transferred over only, which would be correct. I go to iM & FT, turn them back on, go back to check settings, emails, etc., and the new number reappears. I then went online to try & deregister online as per apple support. I filled in the small form with the number I wanted to deregister then you have to press send code which should then come from apple to confirm who I am but the code never comes through. Ive tried this with both iM & FT turned on & off. Can anyone out there help me please?