T elephone signals abruptly collapse as we descend into a bunker at a secret location close to Ukraine’s front line with Russia - for a meeting with one of Kyiv’s most ambitious, controversial and successful military commanders. Easily within reach of medium-range Russian drones and jets , the caution was routine. But it takes on added drama when it emerges that an alleged Ukrainian traitor, working with Russian intelligence, was recently in the area tracking the movements of Brigadier General Andrii Biletskyi. The spy’s mission was to give real-time targeting information to Russian drone pilots when Biletskyi was visiting a front line location - or to find him in a command centre and send the coordinates to a Russian bomber jet, according to Ukraine ’s State Security Service, the SBU - also known as SSU.…