Sub-Wave Network

From TARDIS Index File, the free Doctor Who reference.

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 to Great Britain's former Prime Minister 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 mother in London. It also found Rose Tyler at Donna Noble's house but she was unable to communicate with anyone since the laptop she was using did not have a webcam.

As Martha Jones' superphone temporarily did not function, Torchwood 3 and Sarah Jane used the power of the Cardiff rift, Mr Smith, and every single telephone on the planet to call the Doctor with the number from the superphone, managing to get through to the Doctor's TARDIS so that he could find where the Daleks had taken Earth.

Upon her death, Harriet Jones gave control of the network to Torchwood.(DW: The Stolen Earth)