My old phone number is showing up on other people's phones as my iMessage number and I can't deregister it

My old phone number is showing up on other people's phones as my iMessage number (and it's not in my contact info on their phone) and I don't have that number anymore so I can't deregister it. If people iMessage my new number, that works but I'm missing messages that people send to my old number so I'd like to deregister it somehow.