Is there any other way to delete my phone number from my old iPhone?

I changed carriers ,kept my old phone number Got a new iPhone and I gave my old iPhone to my brother but he keeps having issues with people texting him since he doesn’t have a carrier and only use it over WiFi When people text him it goes to my phone. I called customer service and they told me to delete my number from the old I phone but when I try To do that it doesn’t let me delete it! is there any other way to delete my phone number from my old iPhone ?

[Re-Titled by Moderator]