The registration process for FonYou is quite simple. You just have to enter your ID number as a username and choose a password. (This process is necessary for security in order to prevent the creation of dozens of phone numbers to be used for activities of dubious legality.)
Once you have created your 'fictitious' telephone number you can associate it with your real number to receive through your iPhone all the calls and messages that would normally arrive at your real number.
FonYou is a quite interesting tool that will save you a lot of headaches when it comes to revealing your actual telephone number. Also, this app makes it possible, for example, to use WhatsApp without having a SIM card.