Can an iPhone be accessed by an outside party to overhear phone conversations without?

I made a phone call about an issue in my neighborhood to city services. Then two hours later, a neighbor was telling another neighbor about my making the phone call. I did not tell anyone about the call. Is it possible this person is able to hear my phone calls? I was in my home and no doors or windows were open. If it’s possible, how do I lock him out? He is a stalker and has been harassing me for 10 years! I have a restraining order and wonder if there is a way to prove he’s able to hear my phone conversations without my consent.