Can i switch my sim card that is telstra which is in my iphone and put a optus sim card in my iphone?

Simply said: can you use another sim in your iPhone?

If you’ve bought your iPhone with a carrier, why not stick to it?

The answer to your question is: yes, you can, but it is illegal. You have to jailbreak your iPhone and install yellowsn0w from Cydia, or you can use one of the available sim-modifiers. (or whatever it’s called, like TurboSIM)

Couple of disadvantages: warranty = void, and when Apple updates the firmware, you have to wait for the iphone-dev team to update their PwnageTool/QuickPwn application.

If you’ve bought your iPhone w/o a contract, you can put in an other SIM like that. :)