902 is a virtual number in which calls are diverted to a real line (fix or mobile).
You don’t need any special equipment (switchboard, systems, etc.), only a fix or mobile line where you receive calls. The diversions to a fix line are free, diversion to mobile cost 0,10 € / minute. In case you're thinking to send all your calls to your mobile number, please, check the 902 Mobile option.
The call cost to 902 is completely carried out by the caller (owner of 902 numbers), you don’t pay for receiving calls in your 902; on the contrary, you get an income for each call minute received.