Unable to access AT&T service whilst in Australia

Hi all,

I purchased an AT&T prepaid $45 a month plan. Whilst I was in the US the service was fine. Now that I am back in Australia, I am unable to receive calls, texts or anything. No matter which phone I place the sim in, I receive no reception bars. Using a different phone, I am able to call my voicemail remotely and check messages though.

I've searched through past posts and this does seem to be a common issue; however, I am yet to find a reason why this is happening or a solution. Any ideas?

Thanks in advance.