but the controller was working with another aircraft on another workstation, where he could not hear the B757 (or the TU154M).
You are Correct on the other workstation ( and the different frequencies ) I should have said "covered" instead of "Blocked". But the end result was the same.
For info to others , the other workstition was only 2,5m away and the controller could follow both transmissions and reply to them, but as the frequencies could not be collapsed together he had to go from one workstation to teh other to transmit. . The problem was that at the time the B757 called, the A320 also called and his call covered the 757 call.
One can find this in the report of the controller and on the communication/time chart published with the BFU report.