Sub-wave communication

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 Smith, with Rose Tyler only watching. The signal was then used to combine the entire Earth's phone systems to communicate with the Doctor by using the rift in time and space. He is then able to home in on Earth's location. (DW: The Stolen Earth)