How to regain my previous phone number back on iPhone?

So I just switched iPhones. The dude never asked if I wanted to keep my number and I didn’t think to ask if it was the same cause I told the dude before him. I got home and I’m messing in the settings. Settings ask if I’m still using the same phone number, I click yes. But when I go to the phone setting or my ID settings, it shows a different number. How do I get my old number back??? Settings asked if I wanted to use it so I have to be able to change it right????

[Re-Titled by Moderator]