iPhone 4S PDP authentification failure

[mtiran]

I have a new iphne 4s, v. 5.1. Voice and wifi work very well. When I try to connect ot my data network I get always "PDP authentification failure" and I can't connect to inertet and so on. The access point is correct and everything in my sim card is correct, following my operator information (I contacted them to verify that). Can anyone help me on how to fix this PDP problem, please? What is PDP by the way? Tx