Is it legal, as an owner of a buisness to pick up the phone and listen to conversations without permission and or warning?

I recently was asked to leave my previous employment for the issue of "Not following directions". That morning he (the owner) fired an employee. I was asked to "Call her clients and tell them that she is no longer employed with us and we can not give out anymore information about it". I followed the instructions as I left message on voicemail. Two clients happened to call back who I knew on a personal level and they were not satisfied with the information given. After second phone call, I was called back in the office and asked to leave because I didn't follow instructions. He listened to the conversations I had and gave no other explaination for letting me go.