Many employers provide their employees with mobile phones to allow them to stay in contact when away from the desk. However one down side to this is that employees will use the mobile phone even when a perfectly good extension telephone is within easy reach. It means the cost of mobile calls are higher than they need to be.
There is a a way to reduce this problem. It is call a private mobile phone network. In its most simple form the solution takes the form of a GSM base station located in the office premises. Through a registration process it recognises the employees' mobile phones and handles the call traffic for those mobile phones within range of the private GSM hub. Here's an example and another. There is some capital cost involved to set up the service, but the savings can be substantial.