I've never heard of a world sim. How does that work in terms of billing if you're randomly using different providers
Simon is right about cancelling the direct debit. I did that once with BT and I received a letter threatening recovery agencies within a few days (even though I was in the right).
I'd probably try again and properly cancel, frustrating as it may be. I've not heard good things about Vodaphone customer services (I suspect they use the BT model
).