IEEE Spectrum published an article about three techniques for hiding information in VoIP calls, thus showing again that bits are bits.

Hiding secret messages in MP3 or video files has been done for many years. From the bad guys perspective, there is the problem that copies of these files are left on many servers when they are transmitted by email for example, and therefore can be investigated after the actual transmission is completed. 

Hiding information in the VoIP protocol itself leaves nothing behind to be investigated.