Last year I had my mobile phone sim unlocked so that I could use it abroad. Since them I have swapped my sim card into a new phone. Will this still work abroad, is it the sim card or the phone that needs to be unlocked?
I assume that what you actually mean is that you contacted your service provider and enabled international roaming. If this is the case, then it will apply to your account and hence the sim card.
Though if you had the phone unlocked so that you could put in a different SIM card whilst abroad, it is the phone, not the card, that has been unlocked.