他生气有道里吗？

He gave me two phones, one need to connect to the phone line on the wall, and one is only extension. My son and I live in the same apartment building. I was wondering if the signal can reach that far from my place to my son’s place. So I just bring to his place to test it. The single is strong. It works at my son’s place. So I just left that phone for him. So he can call someone without use his cell phone minutes.
He is mad about this. He said my son could hear our conversation. I said nobody interested our conversation. I pay the phone service bill. It is not his business.
what do you think?