Nokia 7600 - the SIM card was rejected when put in an unlocked 7210

Hi, I have a Nokia 7210 through Orange which I unlocked succesfully about a year ago (tested it with my friends Vodafone sim and it worked). However, today I got a Nokia 7600 from '3'. I got this phone as it had a great contract 500 free X-mins for £25/month - I don't want the phone though, I'd rather use my old one and stick the new SIM in it.

I am trying to put my new sim-card in the 7210, but when I did this the screen read "sim card rejected". Is there a way I can unlock for the '3' network? I'm assuming that '3' is a new company, so when my phone was unlocked over a year ago it didn't unlock for this service provider.

I am in the UK if that helps.

Thanks!

I was told by a representative of phones4u that they use different SIMs to conventional 2g networks. How much faith you put in that information is up to you.
I, like you, wanted to do the same, "chuck" their phone and use my existing handset that has a battery that will last more than a couple of days. I would effectively be using o2 but with about 3x the free minutes.

Paul