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).
If you receive your phone before your network billing date, you get given a partial allowance so you can start using it straight away. This partial allowance is chargeable, and will appear on your first bill.