Polish Researchers have discovered a clever way to send secret messages during a phone call on Skype. They find out this new way to communicate messages more secretly by using silence. Skype sends 70-bit-long data packets instead of the 130-bit ones that carry speech. By taking advantage of this they hijacks these silence packets and then inject encrypted message data into some of them. The receiver on other end will always simply ignore the secret-message data, but it can be decoded back to receive that secret message.
Source: https://thehackernews.com/2013/01/skypehide-to-send-secret-messages-into.html