Old number attached to imessage

I disconnected my cellular service with Softbank in Japan. I had my device unlocked before doing so, because I intend to use my current device with my next carrier. I currently have no carrier and am only using WiFi. When I use iMessage to talk to friends and family it still shows that I’m texting from my old Japanese number even after they’ve deleted the number from my contact. I’ve gone into settings for iMessage and iTunes to try to delete the number as a method of contact but the number is greyes out with a check mark next to it. Please help me delete this number so it will no longer be associated with my Apple ID.

I googled the problem and the only solution I found was to turn iMessage and FaceTime off and back on again, which I did multiple times. I’m still unable to delete the number.