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 and space, enabling Rose to call her Mum while on her adventures with the Doctor. (DW: The End of the World) It even worked for Adam Mitchell, allowing him to phone his mother in his own relative time. Unfortunatly, Adam used the phone in an attempt to bring knowledge of the future back to his own time, for self gain. This led to him being expelled from the TARDIS by the Doctor. (DW: The Long Game)

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)

Due to the danger at hand, the Doctor also altered Barclay's phone in order to get in contact with UNIT. What happened to it afterwards is not made clear. (DW: ''Planet of the Dead)

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), in order to contact him should he ever be needed; 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)

Strengths and Weaknesses
The superphone has the ability to call the person you're looking for no matter where in time or space they are, refered to as "free roaming", and even locks onto the the user's relative time (would you normally have been in 2004, the phone will call from that time); the only exception to this last rule is the Doctor himself. 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, or that the blackhole was running interference. Martha also found trouble contacting the Doctor when the Earth forced out of sync with the rest of the universe, as she found that her phone couldn't contact the superphone(DW: The Stolen Earth).

Also, when the Doctor modified Donna's phone, he was able to call Martha on it depite her being on another planet than Earth and the phone she was using being an ordinary cell phone.