How to change default dialer app in Asus Zenfone 5?

How can I change the default dialer app in Asus zenfone 5. I tried Settings → Apps → All, then selecting the phone app. There was a Clear defaults button which was greyed out. Do I have to get root access to change it? The purpose of it is to automate a conference call using uiautomator and adb shell. For this reason we are supposed to have a same UI for all devices so that we can automate it easily. But right now, even when I call with Truedialer I get this default screen that I used earlier. The resourceid is *com.asus.asusincallui:id/holdButton*. (Click image to enlarge) " class="img-fluid rounded" style="max-width: 100%; height: auto; margin: 10px 0;" loading="lazy">