Currently number porting only works for French onoff numbers but full support will be coming soon.
To port your number into the onoff app, simply select 'number porting' from the main menu and you will be prompted to enter your phone number + RIO code and to select the date you wish for the number to be ported.
The RIO code is a code given to you by your network operator.
To port out an onoff number, simply dial 3179 from the onoff number you wish to port, an automated message will tell you your RIO code and it will also be sent you in the form of a text message. Now you may provide this code to your new telecom operator.