AT&T and South African MTN

To answer your original question no.

AT&T phones are locked out of the box and will only work with an AT&T SIM card. So inserting your SIM card will not function

Your iPhone may work with an AT&T SIM card as long as it is unlocked and will use another carriers SIM. Of course that means you will not be using your Australian phone number. AT&T has prepaid talk plans with 5 GB of data for $30

iPhone 6 or newer will work on AT&T. If you have an older iPhone it cannot work on any US carrier anymore.

why would you buy an at&t cellphone in south africa? buy directly from the manufacturer 

Am in the US currently but I want to buy an AT&T cellphone

We want to make sure you're fully informed on your decision on purchase an AT&T phone in the US, Sandile.

If you want to purchase an AT&T phone in the US and bring it back to your country, you'll need to make sure the phone unlocked. You can check to see if your IMEI (the unique number attached to your phone) is compatible to our network.

If you don't know your IMEI number, there is a resource to help you find it. 

We're happy to do business with you, and we wish you the best of luck in this endeavor. Thank you for choosing AT&T.

Tony, AT&T Community Specialist. 

I'm moving this thread to the Wireless forum.

I’m currently in Utah America I came with my unlocked iPhone from my country South Africa on MTN SIM card but now it’s showing AT&T as my network well I can’t make calls or texts with it

please explain how to make it active 

@BeautyMokoteli You need to contact your South Africa carrier for help. Your iPhone should work unless it is too old. You need an iPhone 6 or newer to use ATT’s network.