Divert is done on the mobile network and does not matter whether your phone is switched on or not, so you will stil incur a charge for the forwarded part of the call, which will be at international rates.
Sorry Keith , thanks for answering, but even I know that information is incorrect, unlless you mean my three mobile would be charged for the international connection? Just off the phone to BT helpdesk and was initally told that by them (both the first reponder and the her manager). After I explained to them how mobile networks work, and that if I call a UK mobile I only ever get charged for a UK call wheter the other phone is in the UK or not, the manger went away and checked then caled me back to tell me that I need to keep my phone off as once it registers on the foregin network everthing becomes an international call. I don't quite believe this euther but wanted to check if anyone has actually done this while away.