My iphone is locked with optus, can i change it to telstra?

I had an iphone 3gs and i didn't use it very often. I lost it for a few months and while it was lost i bought myself a samsung galaxy y. The other day i found my iphone packed away in a small box. My phone now is with telstra and i love telstra because of all the free talk and text caps and i'm not willing to change to optus so i could have my iphone. I was wondering if i could take my iphone into any shop that will change it and make my iphone compatible with telstra and how much it would cost.

You can get the phone unlocked and use on Telstra. Check online for factory-unlocking iPhone which will unlock your iPhone permanently.