Av Callao was a major street in Buenos Aires that was on a map shown to the CIA when Rex Matheson and Esther Drummond were thought to have died in a suicide bombing. The street coincided with Av Santa Fe, and contained Wilton Palace Hotel. (TV: The Blood Line)
Real world location
Though it's possible that a real world location doesn't exist in the same geographic space in the Doctor Who universe, such cases are few and far between. Thus, the map at right is probably a good indicator of the DWU location of Av Callao.