If you want absolute certainty when in the outback, then a PLB is what you need.
If you want the ability to actually talk to someone and not just put out a distress beacon - then Satphone.
If you want a very good chance of being able to talk to someone from anywhere without having to pay for a satphone service, then happyhunter is on the money - HF. There are different bands that are more effective depending on the time of the day, etc.
For instance I'm normally guaranteed a hit somewhere in Australia if I'm using around 7,000hz range. (7khz)
Alternatively, many times I can bring up a New Zealand station if operating on 14khz (14,000hz) and it's not uncommon to be able to get Africa, Europe or America. (These are with a mobile whip on a car, but I've heard of people who have done it with hand held units as well).
I haven't played with CB frequencies that much. It's my limited understanding that once you get above a certain frequency that the chance for 'skip' becomes more dependent on atmospheric conditions and whether the 'band is open'. In vary rare situations it's possible to talk hundreds of km's on UHF as well with ducting.