O2 top ups of �15 a month=unlimited calls and txts to o2 numbers?

im with that tariff or watever that gives you unlimited txts and calls to other o2 numbers if i top up �15 a month, I was wonderin what happens if i top up �15 twice in the same month,wud it carry on to the next month automatically? or does it not count? If i need more credit should i just top up �10 or does it need to be �15? please help