Elite Pro to ring original doorbell

Hi. I´m trying to make Elite Pro to ring my original system (Hyundai HDP200, two handsets and one door station) in order to make them work in parallel. If I connect directly one handset to Elite Pro it makes it ring. But when I put it in parallel with both handsets and the door station (teh current installation), it doesn´t. If - with or without Elite Pro connected- I short circuit the pair, the handsets ring. I thought Elite Pro just did the same (short circuit pair when the button is pressed) but seems not to work that way. Do somebody know, if not contacts, what type of output it has ?. Otherwise, if someone can suggest a way to just make it work, I will be grateful.