Android TV
Android TV is Google's operating system for smart TVs and streaming devices, it's currently the only supported operating system for Engage Call, you can either purchase an Android TV with that system pre-installed, or you can purchase an Android TV stick/box which can be connected to a non-smart TV to add in that functionality.
Getting the Engage Call App
The Engage Call app can be found on the Google Play Store on compatible devices, simply navigate to the Play Store on your device and search of "Engage Call" from there it should be straightforward to download and run the app.