Unlocking a iPhone 4S from Telstra?

I'm buying a iPhone 4S from Telstra and you want to use it on a Vodafone plan or pre - paid and I was wondering how and does it cost and do I ask them to do it when I buy the phone or?

Maybe this will work:
http://crowdsupport.telstra.com.au/t5/Apple-iPhone-KB/Unlock-your-Apple-iPhone-from-Telstra-Post-Paid-Customers/ta-p/65930