You decide what you want to use your monthly plan for each month. A lot of surfing, fewer calls. Great. The opposite the following month? Also great. You don't need to do anything as your monthly plan adapts automatically to your needs.
Each month you have a fixed amount in your mobile plan for calls, surfing and texting. And if you haven't used it by the end of the month? No problem, just carry what's left over to the following month**.
Are you using more than your monthly plan? Don't worry, you'll get the extra gigs and call time at the same advantageous rate as your monthly plan. And texts are unlimited* anyway, just like BASE to BASE calls.