Best answer by Gregor
Ah... Now I follow.
If you are using Pay Monthly deal when any extra charges will be added to your monthly bill. Let's say if you pay £5 a month for your plan and you will make calls to USA worth of £5 then your next bill will be £10. Just remember to adjust your Cap to amount suitable for what you need.
If you are using PAYG then you would need to top up your balance amount that will be sufficient for a call to USA. If you are using PAYG then you don't need to stick to the same amount every month you can top up higher balance if you need it.