I have received five 'one-time' codes in the last 24 hours. All are 4 digit codes and sent twice. Nobody in the family is trying to access the BT account.
I tried to send one to the BT recommended text number 7726. It was rejected instantly without explanation.
Any clues?
(I am not on BT Mobile or EE but recently did make an enquiry at the local EE store about transferring my mobile provider. My email address was requested and I have since received a handful of suspected phishing emails to do with my supposed EE mobile account. I am still with Vodafone!)
there is a problem with BT mail and there are numerous posts like yours
https://community.bt.com/t5/General-email-queries/Email-pin-text-message-barrage/m-p/2472192#M124966