Purpose
The Voice Interface was an application of the Doctor's TARDIS.
It enabled him to interact with a hologram - offering medical analyses and limited psychological counselling.
Features
The Voice Interface could be activated at a simple spoke request, most probably from only the Doctor himself. But the main feature of the interface was the holographic avatar. It could take the form of anybody of the Doctor's choosing, accessed from the TARDIS' memory banks.
Known Interactions
The interface was known to have appeared as the Brigadier on at least one occasion. (AUDIO:Zagreus) When the Eleventh Doctor was poisoned by Melody Pond, he accessed the voice interface as he was too weak to reach the console. It first took the form of Rose Tyler and then Martha Jones and lastly that of Donna Noble, but he dismissed all these avatars, as they made him feel guilt. The interface proceeded to take the form of a young Amelia Pond. This version of the interface informed him that it was the poison of the Judas tree and then spurred him into action, with the phrase 'fish fingers and custard'. (TV:Let's Kill Hitler)