There are two types of charges on your BT bill which relate to different periods:
These are charges which you pay at a fixed amount each month or quarter, usually for your line rental or broadband package. They're applied in advance and start the day your bill is produced. For example, if you pay your bill quarterly, your bill on 1 August will include line rental or broadband package charges for 1 August to 31 October. If this is your first bill, you'll also see charges from the date your service started, to the date of your first bill.
If you're a phone line only customer, your line rental charge is shown on the bill in the 'Phone Line' section. If you have a Broadband and Calls package, it's included in the package price.
These are charges for any additional usage, (such as calls you’ve made or On Demand TV you’ve watched), outside of what's included in your line rental, broadband or TV package. These charges are applied to your bill after you’ve made those additional calls or watched the content. For example, if you pay your bill quarterly, your bill on 1 August will include any usage charges from 1 May to 31 July.
If you've signed up to a new product part way through your billing period, your bill may be higher than you expect. This is because it will include a charge from the date the new service was started until the date of your last bill, as well as advance charges for the next three months.