Google Nexus 5 — How to fix the problem with home screen of Android 6.0 - Marshmallow

Instead of my Home screen with all icons, my new Android Marshmallow run the "Phone" app on the top of it every time when I unlock the screen of my phone. It really bothers me since phone app is not my main program to run when I get into my Nexus 5 phone. I'd like to see only my home screen where I can choose which program to run myself, every time I unlock the phone. All previous versions of Android have never blocked my home screen by any app running in the top of it... What should I do to block the phone app from starting up automatically on the top of my home screen??? Please, review and advise. Thanks