Superphone

The superphone was the nickname Rose Tyler gave to the mobile phone that was altered by the the Doctor using "Jiggerypokery" (which he claimed, mostly in jest, that he "came first in").

It has the ability to make calls through time, enabling Rose to call her Mum while on her adventures with the Doctor. (DW: The End of the World)

The Doctor also altered Martha Jones' phone in an entirely different way using (his sonic screwdriver rather than a technological addition to the phone) to function in a similar manner. (DW: 42)

Donna's phone was enhanced later by the Doctor as well. (DW: The Poison Sky)

Examples of Use
Following her ordeal during the "Year That Never Was", Martha Jones gave her phone to the Doctor (DW: Last of the Time Lords); he kept it with him and was called (by Martha), bringing him back to Earth to assist with UNIT's investigation of the ATMOS facility. (DW: The Sontaran Stratagem)

Martha's phone was also used by Torchwood, Mr Smith and Harriet Jones, all utilising the Cardiff rift to contact the Doctor when Earth was teleported by Davros to the Medusa Cascade. (DW: The Stolen Earth)

Problems
The superphone has the ability to call the person your looking for no matter where in time or space they are, it even works in areas of no service as proven when the Doctor, Rose and Harriet Jones used Rose's superphone to communicate with Jackie Tyler and Mickey Smith while trapped in the cabinet room, something that is usually impossible. (DW: World War Three) However Rose could not get any reception on Krop Tor (DW: The Impossible Planet) implying that the process is somehow related to the TARDIS.