If your handset is locked to a particular carrier (other than Telstra) you will need to contact that carrier directly. Depending on how long you have had the handset you may need to pay a fee to have the phone unlocked.
For Telstra Apple iPhone customers other than Iphone 5c, 5s, 6 and 6+ wanting to use a Boost Mobile Pre-paid SIM in your phone, you will need to unlock your phone from the Telstra network. Apple iPhones purchased from a Telstra Pre-paid offer are subject to a $150 unlocking fee. iPhones purchased from Telstra on a Post-paid plan can be unlocked free of charge. please refer to this http://community.boost.com.au/t5/Setup-Device-Support/Using-a-Telstra-locked-iPhone-with-Boost/ta-p/5653