I made such switch on my 551C. But I use ringing generator with DC12V supply (from lamp/buzzer AC10V via a rectifier). So in my case it is just switching 1 contact to where the AC10V should come: to buzzers loop or to feed the ringing generator. I wired this switch to a control relay in the PBX, so I can switch buzzers to ringers and back remotely from any station by dialing the relay switch code. Alternatively it can even be controlled automatically via timer. But I do not need such.