The Doctor explained to Rose & Jack at the end of "The Doctor Dances" about the Chula ambulance: "This is a battle-field ambulance. The nanogenes don't just fix you up, they get you ready for the front line. Equip you, programme you." Rose then realised, "That's why the child's so strong. Why it could do that phoning thing," & the Doctor confirmed that she was right.
Captain Jack's term for "that phoning thing" was "Om-Com" & with it he could "call anything with a speaker grill." Thanks to the Chula nanogenes, the child could, too.