Skyguide phone system was down as well as conflict warning system
From AP today at 13:32 CET:
Not only was maintenance work being carried out on the software of the Skyguide radar systems at Zurich ACC, but also on their telephone system. This latest insight comes from the German BFU (Air Accident Investigation Bureau).
The duty controller was thus working two radar screens and two radio frequencies on the night of the mid-air collision. On one frequency, he was handling an ILS approach to Friedrichshafen airport, on the other he was coordinating the two doomed aircraft.
Between 23.25.43 and 23.33.11 the duty controller tried in vain to contact Friedrichshafen airport by phone. Immediately after this (at 23.34.49, and 44 seconds before the mid-air collision) he radioed his first set of instructions to the TU-154 crew to descend.