Real-time synchronous communication ( like phone calls ) is pretty hard to hide, it uses bandwidth and takes up space on switches and needs low latency to work at all ( make a voip call to someplace that only has satellite internet to see how bad it can get ). If you're serious about hiding your communications; either build physical networks that you control and limit the amount of traffic visible to the public network ( i.e. trunking calls ) or send messages steganographically encrypted in pictures of your cat you post on Facebook.