Hook up Ring 3 to trigger Panasonic phone system with relay

I have a commercial Panasonic KX-TD phone system in my house that was placed in lieu of an intercom system. The current mechanical doorbell (actually door phone unit) connects to the phone system and rings all extensions when the momenatry switch is pushed. I just bought a Ring 3 but connecting these two systems is something I haven't seen with a google search except for one brief mention. I was planning on hiring a phone guy to complete the connections at the phone system but wanted to determine the design and get the parts since he had no experience doing this. Assuming this is somewhat like how people trigger an intercom, I would need to install a 24V 40A transformer, a 24VAC relay and possibly a 25 Ohm 50W resistor all wired in a loop and additional wires coming off the relay would trigger the phone system ring. Since there is a relay in this plan, do I need the resistor too which is required when nothing else is used? Is there any reason to run a power cord to a hard wired transformer rather than use a plug in transformer? Has anyone done something similar and can recommend specific components?