Chris, you are normally 100% right, but not on this occasion.
"Whenever you use a UK-registered mobile, all calls are automatically routed to the UK. (Even if you call someone in the next room to you in Australia, your call will still be sent via your UK phone company, and you'd need to enter their number as if you were calling from the UK). So, when using YOUR OWN MOBILE PHONE, you simply enter your NORMAL phone number, with no international codes and without omitting the initial '0'. (i.e. start with 017... , exactly as if you were calling home from a friend's house in the UK). "
No, that doesn't work. Been there - done it. You STILL need to access the international dialling system of the country you are in, and give the code of the country you wish to dial, using either "+44" or "001144" in the case of Australia calling the UK. I had to change all my mobile speed dials to add "+44" when I first went out to Oz back in 2000 (and a handful of times since),
Same anywhere outside the UK - if you don't tell the local network you want to dial internationally, by using their access code, then it won't work.