I'm having the exact same problem because the SIM I used for my original Cathay account number is now inactive. I'm able to get through to my account using membership number but I can't do a lot of things due to the need for the number. Worse still, you can't even change the number without the old number, and it doesn't accept other forms of certification like email (because I never managed to get them to be recognized as "official" due to tech issues.) And trying to get through too many times locked me out of the account!

I assume the extra security has to do with the recent hack that resulted in some people's Asia Miles being stolen. Still, I wish it were easier to fix (e.g. via WhatsApp and not phone). I was only able to get through to someone on the busy hotline once (and even that required saying I wasn't a member on the touchtone portion of the call, otherwise they still want to send an OTP to the old phone number), and they told me their systems were down so they couldn't process a number change.

Going to wait a few days since they must be busy with flight cancelations due to the recent typhoon still and see if I can fix it then, but if anyone from CX sees this *please* figure out a way not to tie everything to phone; even low tech banks in HK will let me fill out a form to deal with this.