mobile phone network lock out

Not very well up on phones but I bought one second hand, all is well,works fine and I've got £17 in credit.
Thing is the previous owner put a password on and it locks out any attempt to set a security code etc.
I was told to change the sim so I did to Tesco but it still says Network lock out.
what happens now?I'm happy to stay with Orange but I wanted a security code and I know you can get them unlocked but the firms on the net want a few quid and although I don't mind that I'm not putting credit card details on their websites.
Phone is a Samsung,any one know how to proceed?thanks if you can steer me the right way.
Thanks for the help mate,thought I'd cracked it but unfortunately my model SGH-G600 was not there.
There was another link but the phone call was £1.50 an hour at nearly 5 mins long.It's not worth that.Thanks anyway.Regards.
If the phone works and then after a time locks itself then it is the phone locking and not the sim. I am not certain but a 'restore factory settings' might do just that (but it WILL ALSO DELETE ALL OF YOUR NUMBERS AND OTHER STUFF), or apart from that you will probably need to take it to a phone unlocker in your local market.
thanks for that,no the phone works fine it's just the previous owner had a password on and it won't return to factory settings without password.
It's all strange to me took it down to Newcastle and an unlocker stated a new sim card would do it but it didn't.
I think I'll run down the £17 left and try again with a cheers.

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