When your contract application is accepted by the network, your billing date (or Direct Debit date) will be confirmed on your first airtime bill. But the quickest way to find out is by downloading your network app (links at top of page). Alternatively, you can contact the network directly to find out.
RPI stands for retail price(s) index, and is a measure of inflation published monthly by the Office for National Statistics. Each year, your Pay Monthly airtime tariff will be adjusted according to the Retail Price Index (RPI) rate of inflation. Your bill will be adjusted in March (EE), April (O2 & Vodafone) and May (Three) by the rate announced in January (EE & Three), February (O2) and March (Vodafone).
That's because you're paying next month's line rental in advance. You're also paying pro-rata from the day you were connected - and that's a one-off charge to get you caught up. Your inclusive allowances will also be calculated on the same pro-rata basis. After the first bill, the monthly line rental will be at the amount that you signed up for.