Download SablePhone
Desktop app
For your computer. Install it once, it updates itself.
First time on a Mac
SablePhone is not signed with an Apple certificate, so macOS stops it the first time. Open the downloaded file, drag SablePhone to Applications, open it once, then go to System Settings, Privacy & Security, and press Open Anyway. You do this only once; updates install by themselves.
First time on Windows
Windows may show "Windows protected your PC". Press More info, then Run anyway. Only once.
Android app to control your phones
For an Android phone or tablet you use to see and control the other phones.
App for the phones being controlled
Install this on each phone you want to control (not on the phone you control from). Sign in with your account and the phone shows up in SablePhone. It updates itself.
iPhone, iPad, or any browser
Use SablePhone in the browser and add it to your home screen: iPhone and iPad in Safari, tap Share, then Add to Home Screen.