Andrew from Vivid Adventures replied:

all pretty obscure and hard to test...

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Big Woody replied:

Hi Geoff,

I think you will find that all along it has been a Telstra issue. All the mobile phones in our family and business are through Telstra (some GSM and some CDMA) and over a period of about 6 weeks, several of the phones had SIM card issues where the phone could not recognise the card. The phone could not make calls but sometimes it could receive them. Then we would put old SIM cards into the phone and they would work OK for a while and then stop.
The problems for most phones seemed to last about a week or so and then they would work again as normal.
Maybe something to do with the roll out of NextG or something but dozens of people I know commented on similar issues.
A week after they were all working properly again I noticed something in our local paper talking about the problems with Telstra SIM cards.

BTW I also have a Motorola 9505a and use the same set up as you with a GSM SIM card with internaional roaming enabled. I use it regularly as I have a farm which is quite remote and I am up there every fortnight or so on my own. As you said the calls are about $3 per minute but what price can be put on safety. On my last trip up there I was quickly reminded of why I carry the satellite phone when I found myself about 15ft from the largest and fattest coastal taipan I have ever seen travelling across my path. I estimate it to be at least 7.5ft long but the bit that shocked me was his body was fatter than my forearm. I see a few snakes now and then but this one really put the shivers up me. With a snake like that I may not survive but at least carrying the satellite phone and GPS for accurate co-ordinates my chances of the chopper arriving in time are significantly increased.

Regards,
Brett