I hope BA gets it right, especially for those of us with non-UK memberships.

Certainly my US-based financial institutions give me multiple ways to complete 2FA - it's usually either SMS, or a phone call, or an e-mail. More recently it's also push notifications to their app which will authenticate web browser access. However, when my UK bank moved to OTP for their website it only worked with UK and Spanish mobile numbers even though, in my case, you could specify a US number. Then, after it worked flawlessly for ages, it broke and would show one of my old UK numbers for OTP when trying to pay with my card, while OTP for website logins still showed my US number. No SMS was received and so I couldn't pay with my debit card nor could I log into my bank's website. The bank acknowledged the issue but couldn't say when it would be fixed. Then it just randomly started working again. Other challenges have been not getting the SMS codes when travelling abroad and having to rely on e-mail instead.

It is actually really frustrating that so many companies still don't support the E.164 number format or, even, numbers outside their own country. For example, Arrow Cars at NCL airport allow you to make a web booking using a non-UK phone number but the kiosks at the airport only take a UK number so you need to either have one of the staff pull up the booking to summon the car, or you need to go inside to the desk. It's one of the reasons I have a Vodafone PAYG number as I simply cannot rely on UK merchants supporting foreign mobile numbers.