SOS only mode

Hi

I left New Zealand and travelled to Australia last week. When I landed in Australia my iPhone5 automatically selected their Telstra cell network. Within two hours though my phone went in 'SOS ONLY' mode.

I could not get my iPhone5 out of this mode and select a cell network. Wifi worked fine though.

Does anyone know why this happens and what the fix is please? I had another mate with an iPhone6 that has just come across the same problem.

Thanks

Tony