If i purchase an iphone 6 on a plan, can i put my own sim in it?

If i purchase an iphone 6 on a plan, can i put my own sim in it? - 1

Added (1). Someone is selling an iphone 6 thats on am plan with optus that i don't want to pay, but if they take their sim card out can i put mine in and not pay their plan? Like will it be my number/my bill & my account with optus?

Added (2). their with optus and so am i so it doesn't matter about it being unlocked

Added (3). i'm in australia we don't have at&t and like i said both optus

Only if its been unlocked

Nope Only the one comes with it. Like you can USE at&t SIM only if you Got at&t IPhone. You can get it unlocked to use another network carrier

As long as it's the same carrier it should work fine.

You should ask them to request an unlock though - most carriers will do it at some point during the contract and only the original owner can do that, with most companies.

Either for future changes or just resale value, an unlocked phone is far more useful.

I wouldn't buy it as it would most likely be locked to Optus and the person may report it lost/stolen or wouldn't remove the icloud lock from the device. I think the risk is too high! Also you may not be able to unlock it for overseas use.