No, before September 2020 the contracts only allowed BT to increase mobile bills by CPI (actually a bit less than CPI because they rounded charges down to the nearest £5). The CPI plus a percentage formula was brought in for contracts entered into from September 2020 onwards. There will be quite a few people on two-year mobile plans with up to six months left to run whose annual increase is capped at CPI, and it’s those people who are affected by this new change and will have the right to early termination.
So quite possibly when you raise the possibility of early termination as your contract does not allow CPI + 3.9% BT just correct the increase to CPI which is the correct increase