If this project goes into life (still waiting for customer's feedback), I will concentrate on reliability first and try to cut cost down as much as possible.
The difficult choice I have is either use microcontroller for each floor or just use some of the devices suggested by friends here using 1 or 3 wire interface which might be cheaper and easier solution to implement.
I like the driver circuit suggested by Kai and the modifications made by Karas.
This project is easy and difficult at the same time with plenty of room to maneuver!
I think I can benifit from the way PSTN works because this project is quite similar but much less complicated, I can make cleaver things then like alternate routes in case of failures etc...
Mahmood