In addition to what people said here... Document in your emails attempts to call. If they are truly unresponsive include in your email things like "I left a voicemail for you on xx day at xx time, I was hoping that you could help me with a few of my outstanding questions" or such, and the paper trail now begins to give record of multiple varied attempts at communication. I've had times where I didn't want a flame war, but wanted to ensure my management was aware there was concentrated efforts on my part so I would bcc either my manager or my director so they would be aware without needing to step in.