I tried other unlock services, tried to unlock the phone via the carrier O2 UK, tried to unlock the phone with the help of the official Samsung support but nothing worked. The problem was that I missinterpreted the original network PIN request with the PIN of the SIM card so I entered the wrong number too many times. Thus the device did not accept the network unlock code any longer, instead it asked for a network unlock PUK. Neither O2 UK nor the Samsung support team were able to help me any further as they wanted proof of the original buyer of the phone (which was not possible to sepcify as I won the phone at a conference). You made me very happy because you sent me the defreeze code as well which was the key for solving the whole puzzle :-) Thank you very much!