Ahh great to know thank you for updating me. My (apparently very wrong ) sources said it was upon network registration.This is what I received from Vodafone when I switched my iPhone on in the USA last week:
"... Welcome to The USA.
Warning, now that you have activated your mobile device overseas, there may be delays in receiving usage information and alerts.
The good news is you can now use your Australian plan while you're here. This means for just $5 extra a day you can make calls to standard numbers, send TXT, or use the internet - all from your regular plan allowance.
This Vodafone offer is automatically activated each day you send/receive calls, use your data, or send a TXT/PXT. For more info or to opt out, free call +61414141414 from your Vodafone Mobile.
Don't forget you now need to dial +61 instead of 0 when calling Aussie numbers.
Happy travels!....."
So, only when you make/receive calls or text or start using data, not when phone is idle with Data Roaming switched off.
Good to know though thank you =)