Hi Ian,
It looks like to me, some sort of old style built in telephone. The built in ones were often bigger then your conventional mobile but I think they are still made by phone companies instead of the car companies if that makes sense. Obviously someone has removed the front part of the holder along with the phone.
Is there a charger or any wiring running to it? (Behind the bracket attaching it to the dash) I ask as the charger might help identify the manufacturer.
Sorry I can't help more, My only experience of a built in car telephone one was on a friend's Volvo 740 gle estate.
Cheers
Steve