Sub-wave communication
From Tardis Wiki, the free Doctor Who reference
A sub-wave communication is a form of communication thought to be undetectable by Dalek scanners and was developed by the Copper Foundation. The wave was used by Harriet Jones to contact Martha Jones, Captain Jack Harkness and Sarah Jane Smithvia video conferencing. Rose Tyler is only able to watch. The signal was then used to combine the entire Earth's phone systems to communicate with the Doctor powered by rift in time and space. He is then able to home in on Earth's location. However the Daleks are now able to trace the source of the transmission. (DW: The Stolen Earth)
