Just go to buy a new one (some is only $99).........Vodafone & Three are merge as one company now so you can transfer your plan to Vodafone if it is necessary......
Three
GSM900 (Telstra Roaming)
GSM1800 (Telstra Roaming)
UMTS850 (Telstra Roaming)
UMTS2100
Best Handset to buy: GSM900+1800, UMTS2100+850
Note: need a UMTS2100 handset as otherwise you'll be constantly roaming.