number can’t be removed from imessage

i changed to a new number and i don’t have that number or the iphone anymore.

in my settings i have two numbers, my old one and my new one. i can only remove the newer one but i want to get rid of the old one because i no longer need it. however, it wont let me. i don’t know what to do because i don’t have the phone or the number anymore and it sometimes shows my old number when i text somebody.

i would appreciate a few tips for what i need to do. thank you!