Built in phone not charging
 

I have a Audi A6 -08 with a built in mobile phone in the armrest. But today i couldn't start it because the battery level was to low. And i thought that it was always charging when put in the cradle? Tried to start the engine if it would only charge with the engine running, but it didn't help.

Does anyone know what i could do to try to find the problem? Maybe change a fuse, but where are they and which one to change? Dont know what kind of phone it is but dont think i have a external charger for it, and it seems wrong to have to charge it that way.