I think my abusive ex is listening to me through our kid's cell phone

What can I do to prevent/disarm?

My coparent gave our 5 year old a cell phone "so she can call Daddy." She's called once or twice, but mostly uses the phone to play games. Is there a bag or other device I can put the phone in to reduce/eliminate his ability to listen in to my home? I think this is happening because he sent me a couple of creepy emails alluding to things I've recently discussed in my home.

call called cell phone device games listening old phone play privacy