If you have sent the 4th letter from the BSR book and the bank hasn’t replied to you in writing by the 28 day timeframe then they have silently agreed to it.
I’d suggest ignoring the texts as they must contact you in writing only as the letters say. I personally consider any text from them after I have notified them to be disrespectful, really they’re trying to weave around however they can, and they’ll say anything to try to hold frame etc
Make a complaint if you wish, But not with your telco because they can’t do anything.
Follow the flow chart in the BSR book
Contact BSR via email for advice regarding the process too, they’re greatly helpful