800 is a virtual number in which calls are diverted to a real line (fix or mobile).
Calling to 800 and 900 is free for the caller.
Currently, 800 numbers are being used instead of 900 in order to homogenize with the rest of the world, where 800 numbers are being used freely.
You don’t need any special equipment (switchboard, systems, etc.), only a fix or mobile line where you receive calls.
The call cost to 900/800 numbers is completely carried out by the called (owner of the 900/800 numbers).
The costs vary by the origin of the call (fix, mobile or telephone booth).