The problem is Your phone shows Vodafone roaming signal.You need to unlock your IPhone4 with telstra network so that you can use your mobile with Vodafone network.To unlock Iphone there are several methods out there among them i found Remote unlocking method is the safe and secure way to unlock IPhone.If you are intersted in this method of unlocking means visit this site Classicunlocking.com.After unlocking you can use it with any gsm networks.