Unlike other smartphones, Nokia has made unlocking the SIM card slot on their phones a little easier than other phone manufacturers. Unlocking a phone’s SIM card slot is important if you want to use your phone with another carrier when you travel or want to switch to a different network (for any number of reasons) without buying a new phone. Otherwise, if you haven’t unlocked your phone or didn’t buy it factory unlocked, you’ll have to buy another phone while traveling or you’ll be forced to get another phone when you switch networks. Making sure your current phone is SIM-unlocked will allow it to be usable on compatible networks.
If you have one of Microsoft’s Lumia smartphones, you can SIM-unlock your phone in three steps once you have the unlock code from your the carrier (which can be requested from them after a few months of service) or can be bought online from a third party for a fee.