We have a T-Mobile contract where we can add international data roaming for a nominal charge. I know that I need to set up my phone so that it doesn't receive any calls, as I will be charged for them.

I managed to do this perfectly on a trip. On the last trip, despite following the same directions, the phone rang at least once. The forwarding instruction seems to drop easily. And if I look at my phone to be sure that all calls are forwarded, the simple act of looking at it causes it to revert to conditional call forwarding. When we returned home, we had a several hundred dollar phone bill. T-Mobile had not properly set up the phone for the international data plan so deducted all charges, whether phone or data.

How can I be confident that all calls are forwarded, and I will be able to use the text and email portion of my phone? The phone agents are not knowledgeable, and I cannot find a FAQ on this topic on the T-Mobile site.

I have a BB 9780.