Attached is a "phone-hold system" that I made for my home, many years ago. I copied it from my old pencil schematic an while back, and I think that the logic labels are correct. The little SCR/LED/Pushbutton circuit in the lower left was in each phone. This required that I used all 4 wires at each phone jack. When the line rings, the LEDs pulse with the ring pulse. When a phone is off the hook, the LEDs are on steady. When a hold button is held down as the phone is placed on the cradle, the LEDs will flash on and off. I still have it, but only one old phone has the original "hold" circuit in it. All the other corded and wireless phones, with their own "Hold" buttons, work with it. And I've switched from Quest to Vonage and it works with the cable box.
There might be some things in there that you can use. I also see that I could have simplified the input and eliminated one bridge rectifier.
Ken