question about unlocking

[flasher33]

hi I am a noob to this and want to ask a question about unlocking.
when buying credits what do i get?
I mean do I need to unlock my modem once for it to accept all sim cards??
or every time when i want to change the sim card I need to unlock?

[Avantha]

Unlocking is only one time,after that you can use any sim as you wish.When buying credits,you will get username and password to connect to the server.