You don’t even need to use an old phone. Useful tablets are available starting at like $40, with much more current OS than your three year old phone.
I just got one for $50. It is functioning as my Smartthings control panel. You pick it up and the screen goes on, and the home screen has Sharptools widgets to manually control the priority things. In the background it is detecting bluetooth presences, and it is in always-listening mode so even when it’s dormant I am saying “ok Google, unlock the front door” and it’s working.
So last night I added video monitoring to it via its front camera. It also senses vibrations… I’m going to make a simple wood stand for it, and have it sit on a table in the corner of my living room.
This is progressing so rapidly that I am seriously considering replacing multiple banks of light switches with such tablet control panels. Given that a single smart switch is as expensive as a tablet, why not simply eliminate the switch leg entirely and control the bulb ‘directly’ via the tablet? Especially when said tablet can perform so many other useful duties.
Not doing all that right now of course, it requires a bit of research and experimentation. But it sure would be an interesting approach.