you need to buy an outright phone if you plan to use another SIM other than the one that's purchased in a prepaid format.
it's gone on for as long as phones have been around. if you buy a prepaid Telstra phone and you have a Vodafone SIM it won't work unless you unlock it.
you should buy a prepaid phone for your SIM, eg .. if you have an optarse SIM card you need to buy an optarse prepaid phone ..etc
like Laurie said, that's how it is ...
matt
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