Because they can. In their terms and conditions.
Inflation is 5.4%. Plus 3.9% so that's 9.3% increase. On top of everything else.
Time to negotiate as I am out of contract on my mobile phone.
www.which.co.uk/news/2022/01/mobile-network-price-rises-which-providers-are-best-and-worst-for-inflation-2/
Mid contract price rises have taken various forms, with some increasing the price by inflation only, and others attaching a fixed percentage alongside inflation. This is likely due to the low rates of inflation in recent years. Last year, ID Mobile, O2 and Virgin Mobile raised their prices just by the RPI inflation.
However, this year only ID Mobile kept this increase. O2 and Virgin Mobile will now increase their prices by RPI inflation (which is 7.5% at the time of writing) plus 3.9%
As with last year, BT mobile, EE, Plusnet mobile and Vodafone are increasing their prices by consumer price inflation plus 3.9%.