My home is 20 years old. Front door was wired only with Cat 5 cable to a Fon box that ring the doorbell via our wired phones. Wired phones are a thinkg of the past. My Fon device only delivered 4 volts. I bought a doorbell transformer (adjustable 10-24 volts) and used the Cat 5 (4 wires for each) and am able to deliver 18 volts without problem. I’m sure someone else has thought of this but I thought I would share it. I used the solid color wires on one and the stripped wires on the other. Immediately upon turning on the transformer the app went from battery 95% to Hardwired.
I just wanted to add a little data to my original post. I did some math on the wire and it appears that each wire in a Cat 5 cable is 24 gauge or 0.0201 inches in diameter. I did find some that says that if the wire is really long runs, there may be cable out there that has 22 gauge wire.
Using 24 gauge, if you multiply that by 4, that is 0.0804 inches diameter which is roughly the diameter of a 14-16 gauge wire. So obviously this is more than needed for the 18-24 volts from the tranformer to the ring but it is a work-around for folks like me who just have a Cat 5 cable from their doorbell.