Calls go to my ex?

Sometimes when my mom and dad calls me, it goes to my exes number. We lived together for like six months recently, but we had completely different phone plans and never logged into each other‘s account on our phones ever. Just today my friend tried to call me and she said a man answered, he texted her, cussing her out and when she showed me the screenshots it was my baby daddy’s number. I don’t get how our phones are interlinked like that? I don’t get any of his calls he only gets my calls.