Do I have to return my cell phone which my ex bf bought me as a gift?

I got iphone as a BD gift from my ex bf. He added me in his family plan and he paid for monthly service. Since we broke up, I have my new phone also, he asks me to return the phone. I thought the phone belongs to me, and he could cancel the service anytime, but he insists unless I pay for the cancel fee for $175, he wants phone back to use for another person(i.e new GF or his kids or whoever). I have lots of my private data in my phone, and I simply dont want to see him to return my phone back.