The majority of contract and pay as you go phones are locked, so you need to unlock them in order to use a different SIM. This is a deterrent to thieves, as it prevents stolen phones from being used easily, but its main purpose is to prevent users from being able to change your supplier.

However, it is not hard to unlock a phone. In exchange for a small fee, most mobile phone shops will unlock it for you. You could, alternatively, get it done over the internet for a minimal charge. Most networks also provide this service for a small fee. Once unlocked, you never need to unlock it again, unless you decide to have it locked for security reasons, and this means that you can use any SIM card you like with it.

One of the main reasons why people unlock their phones is that you can take advantage of cheap local SIM cards while you are abroad, or if you need to make international calls. When you change your SIM, your phone number will also change, but if you plan to stick with your new SIM card it is not impossible to change the phone number over.

You can now get SIM cards that are pretty much dedicated to the purpose of making international calls cheaply, and this can be much more convenient than buying an international calling card or using VoIP services such as Skype. When you are done making international phone calls, you can put your original SIM back in, and your phone will behave exactly the same as it did before, with the same phone number and tariff.
Changing your SIM when you go abroad can be a great money saving idea, as this gives you the opportunity to take advantage of cheap deals in the country you are in, whereas if you stick with your old simcard, calls and texts will be very expensive. While this may not bring down the cost of overseas calls very effectively, it can make processes such as phoning taxis, friends, or restaurants while you are abroad a lot cheaper.

This guide to using your phone overseas was penned by Jason Jansen. Cheap international calling is available at Lebara Mobile.