Most mobile phones sold in Australia are locked to the network that they were originally sold through, although a small number of phones are sold already unlocked. If your mobile phone is on a contract there is usually no problem getting your phone unlocked by simply calling your network and asking them to unlock it. Mobile phones on prepaid plans are usually locked to a network but if you have been with the same network for a while it can usually be unlocked by calling your mobile phone company. Pre-paid phones on Boost, Optus and Virgin Mobile can be unlocked after $80 of credit has been added to the phone.
Double check that your phone has been unlocked even if Optus or Telstra tell you that it is unlocked.
The best way to test this is to insert a SIM card from a completely different network to see if your phone will work on a different SIM card. For example you can test that a Telstra phone is unlocked by inserting an Optus or Vodafone SIM card. It must be a SIM from a completely different network and using an Aldi Mobile or Lycamobile SIM is no way to test that a Telstra phone is unlocked (Aldi and Lycamobile use the Telstra network), likewise don't use an amaysim or Virgin Mobile SIM to test that your Optus phone is unlocked (amaysim and Virgin use the Optus network) and don't use a Lebara SIM to test that a Vodafone phone is unlocked. You must test with a SIM card from a completely different network.
As long as your international SIM card is active you will be able to your this SIM to test that your phone is unlocked, however if you do this it is important to turn off data roaming to avoid any roaming charges. O2 UK SIM and T-Mobile USA SIM cards (on the Pay By The Day plan) are sold pre-activated and can be used straight away to test that your phone has been unlocked. T-Mobile USA SIM cards (on a monthly prepaid plan) can be used to test your phone once your activation date has passed. However 2degrees New Zealand and Lebara France SIM cards cannot be used as these are not active until you arrive at your destination.
Unfortunately both Optus and Telstra staff have been known to regularly tell people that their phones are unlocked when they are in fact still locked into the Optus or Telstra network. This is often the case with iPhones as these may show as being unlocked when checked by Apple, Optus or Telstra, however these are not unlocked unless you have completed the unlocking process by restoring your phone in iTunes on your computer.
Call 1300 555 002 to unlock a phone on the Optus network. If you have an iPhone on the Optus network you need to call 1300 075 395.
More information on unlocking phones on the Optus network
Visit a Telstra Store or call 125 8880 to unlock a phone on the Telstra network. If you have an iPhone on the Telstra network you need to start the unlocking process online at https://unlock.telstra.com/.
Call 1300 650 410 or visit the unlocking section of the Vodafone website to unlock a phone on the Vodafone network. There is a charge to unlock a Vodafone mobile.
Phones on with Crazy Johns Vodafone can be unlocked here
Call 1300 555 100 to unlock a phone on the Virgin Mobile network
More information on unlocking Virgin Mobile phones
Please note that when you call your mobile network to unlock your phone you need to have the mobile you want to unlock with you and you need to call from a different phone to your mobile. You will most likely be asked for your phone’s IMEI number, which is usually printed under your phone’s battery.
It is more difficult if you are already in the United States, as many American mobile phone shops don't know how to unlock phones, especially phones on a foreign network. However there are several websites where you can generate codes to unlock your phone. These sites include Trycktill and Unlock it Free. If you are in the United States you can also try making an international call to your home mobile company to get an unlocking code, but occasionally you need to be calling from within that network's coverage area (depending on your mobile network at home).
You can read more about unlocking your mobile phone here.