its a number that joins your original mobile number.
and its guaranteed not to change for life
so you could be on any network and have it linked to an 07077 number and thats what people ring to get you no matter what phone its linked to
Network code is used your mobile by changing one to another network...if you need to change your network means you have to get unlock code....just you try this http://www.unlock-free.com/ link and get the unlock code for your mobile at free of cost....