Max Voltage

I have a Koch K93 UP. Unfortunately it is not officially supported by ring.So i used the brand "Generic" with the type "6-Wire".There is a pin 5 with 28,4 V (measured) on the intercom which is shorted to 9 to open the door.So i just hooked up A-1 to the pin 5 and A-5 to pin 9.This works.So basically ring intercom just acts as a relais.But there is no statement about the voltage. What is the max voltage ring intercom can handle?