The socket could be for the Nokia hands-free kit you mentioned I suppose, seems likely anyway.
My Mondeo has a 3.5mm auxiliary socket for the stereo in the glove box, so you can connect an mp3 player or something, I don't know if the pre facelift Mk3's have them or not, I guess it could possibly just be an auxiliary socket if it's not for the phone. Although the location seems very odd if it is.