There are foreign numbers which calls are routed to a real line in Spain . You could get numbers in France, United Kingdom, Swiss, Romania, Argentina, Brazil, Chile, Colombia, Guatemala, Mexico, Panama, Peru, Venezuela, etc.
It does not need special equipment (switchboard, PBX, etc.), only a fix line or mobile where you could receive calls.
Calls on international number do not obstruct calls coming in your usual number.
You could receive calls from your international and usual number (your phone will ring if someone calls you to your both numbers).
It applies only to received calls, it does not act on emitted calls which will be realized by your usual operator.
The cost of calls on your international number is taken on, in full, by the customer whose call, you do not pay receiving calls on your international number.