Make a dated schedule
List the beginning of the agreement, any promotional expiry, scheduled service change and minimum-term end. Add separate device and subscription dates. You do not need a sophisticated spreadsheet: a chronological list often reveals a change that a headline amount misses.
Identify the reason for a change
A bill may increase because an introductory reduction ended, an agreed scheduled change took effect or extra activity occurred. It may decrease because of a one-off credit. Compare the description and period on the bill with the contract before assuming the new total will repeat.
Ask the right question
Ask the provider which contract provision or usage event produced the change. Request the itemised explanation and the options relevant to your agreement. Current adverts and a friend’s contract are not reliable substitutes for your own documents.
When comparing an alternative
Count each distinct period separately. Include obligations that survive a switch, such as a device agreement where applicable. Do not assume a number transfer erases the existing balance. These pages explain the method and do not maintain a live tariff or fee catalogue.
Source notes
Confirm the current detail with the official provider. Links are references, not partnerships.
Ofcom consumer information ↗