I'm on orange and I have the same problem as far as I can gather you just get extra minutes for purchasing a 3g phone, I think it's just an incentive, not sure if there are other conditions attached.
My actual talk plan is 300 minutes, but I also got 300 extra minutes, and 100 extra "3G" minutes on top of that.
Giving me a total of 700 per month.
However, when I call to check my allowance usage it says i have 300 minutes, and counts that down to 0, but after that, as far as i know, there is no way of knowing when I have used my other 400, so you need to be a little careful.
I've had my phone two months and both months have gone over 300 mintes, so the thing tells me I have 0 minutes remaining, but on every bill because it's not been over 700 mins I havent been charged... so you need to vaguely keep track of what you're using each month.
Any orange subscribers here know of a better way of doing this?