I don't mind. They never really describe what the 'calls' were like in the books. I always assumed it was like a telepathic link through the Dark Mark. Though you do know that if Bastien goes with him, that Voldemort will have no problem outing them to each other. As in him calling Barty by his name in front of Bastien and possibly doing the same to Bastien for having the guts to go see him face to face for the first time. :P