Yeastar S20 -incoming calls

I have recently installed a Yeastar S20 with 1 SNOM D785 and 7 SNOM D735 Phones. 

Theres is now a problem: If the Yeastar is not accepting directly the incoming calls, from two incoming calls one is finished after 15 seconds without taking it up. The other is doing fine. If the yearstar ist taking up directly without ringing of the calls, two incoming calls are working. 

What could be the problem?

Thank you

Markus Koch