i can suggest you that there are two kinds of solutions available ie switch based and server based.
Depending upon the requirement and investment potential you can chose about switch based or server based solution.
Server based solution is ideal for 24 to 400 seater set-up and is available at attractive prices and now a days ASP model is also available ie leased or rental model on per seat per hour for a fixed hours in a day.
I know your locations may i have the idea of how big your intended call centers are going to be?