Sub-Wave Network
The Sub-wave Network was a piece of sentient software programmed to find anyone who could help to contact the Doctor. It used sub-wave communication to transmit, which means it is undetectable as it is below normal waves. It was created by the Mr Copper Foundation and further developed by Harriet Jones.
The network found Jack Harkness at the Torchwood Hub, Sarah Jane Smith at 13 Bannerman Road and Martha Jones at the house of her moth. It also found Rose Tyler at Donna Noble's house but se was unable to communicate with anyone since the laptop she was using did not have a webcam.
With Torchwood 3 using the Cardiff rift, Sarah using Mr Smith and using every single telephone on Earth to call the Doctor, they managed to get through to the TARDIS. (DW: The Stolen Earth)
