In one Soviet film I had seen a Soviet officer who was in Hitler's bunker tasked with maintenance of a telephone line between the bunker and the Soviet command.
But this was about the final stages of the war. I wonder whether something similar was undertaken earlier.
Almost certainly this was a bunker repurposed by the Soviets as a high level command center, the phone line put in as part of that.
While it's not impossible that there was a direct phone line between the German and Soviet governments at some point early in the war (there were quite a few economic links between the countries of course, and German military training happening illegally in the USSR), they didn't exactly see eye to eye after the start of operation Barbarossa (and there was little love lost between them even before that) so I don't see Stalin and Hitler sitting down with a brandy and cigar in front of their respective fireplaces and having a nice little chat over the phone.
Most likely though the whole thing is nothing but fantasy, at the time late in the war a radio link would be more likely, and earlier it'd have been telegraph, not telephone.
They might have installed a telephone line to arrange for the May 1st meeting between Krebs and Chuikov to negotiate a surrender of Berlin garrison. Hitler was already dead and Goebbels died that afternoon, opening the way to the surrender.
