Authentication code?

[Guest sqd]

So, a pretty long time ago, I updated my i900 with a sector WM6.5 rom. After that I couldn't recieve MMS since the settings were incorrect. I didn't bother fixing it, since I usually don't recieve MMS. But I decided to just do it, since it's (supposed) to be very easy. I just send a sms to my provider, they automatically send over the settings. But when I try to install the settings they send, im told to put in a "Authentication code", what code is that? All I've tried is the PIN, and that's not the one..