Frequency Change
My initial thoughts here were (as reported as a possibility on Sky News) that the crew of the Tu154 had changed frequency before the warnings came from the German ATC. Did the Tu154 crew respond visually to the 757?
The warnings I assume came once the flightplan of the 757 had been handed forward from Swiss control. Please correct and forgive me if this is an unlikely cause.
Deepest sympathy to all those close to the unfortunate vicitms of this crash.
FS