Unlocking phone - do I do this with old sim card or new sim card in my phone?

So I was using vodafone with my blackberry, I managed to get the NUC code and everything to unlock my phone to put in a new sim card from giff gaff

Do I do this NUC code with my old sim card in my phone or with the new sim card in my phone??? (or does it not make a difference?)

thank you :)
 
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