first of all
to the site
have a look on your last bill to see when your billing cycle runs from, it will be something like 12th - 11th.
if you were to add it on then you would be charged from today to the 11th and then one full month 12-11th so could in theory can charge upto 2 months. in this example would charge 19th march till 11th april and then 12th april till 11th may
when you ring back to have it taken off, any charges you have paid in advance would be credited back to your bill so if you rang back on the 19th april you would be credited 19th april - 11th may
good luck
