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You can use your SIM in another phone if (a) the other phone is unlocked for use on any network and (b) the SIM is the right size (standard/mini, micro or nano).
If you insert it into another phone and it prompts for a SIM PIN to unlock the SIM then you just enter the SIM PIN (PIN1) or PUK if necessary:
I forgot my PIN... What now?
If the phone says something like "invalid SIM" or "Enter NAC/network unlock code" etc. then the phone is most likely locked to another network and needs to be unlocked.
Hope this helps.
Did that help at all @gerbiles?