It appears they are trying to lie their way out of it...
Which frankly, is utterly shocking.
Their own T+C's make it abundantly clear as to which period the RPI comparison is made, and now they are trying to weedle out of it, with straight up lies.
The latest response indicates that they 'guessed' at what the RPI figure would be. Is that a responsible way to run a publicly traded company? Is it possible even against the law?
If they got it wrong, millions of punters would be entitled to cancel. Well, I suoppose they already are.
Tmobile have screwed up, and instead of admitting it, keeping it quiet, and handling the small number of people who spotted it, they haveput up the barricades, and allowed it to balloon.
Fact, they have screwed up. Their OWN T+C's dictate very clearly their own allowed actions.
They have messed up.
FYI: Anyone that wants to take advantage of this (and given how Tmobile have behaved, I strongly recommend that you do) you must do so soon... Before the rise comes into effect.
I suspect this is the reason they are stonewalling. Once the rise takes effect, the rise is binding, even if they got it wrong. Once the rise happens, I think you will find them quietly admitting the mistake, and trying to sort out deals with customers. So get your complaint and cancellation request in pronto.