Nedbank cant change a cell no

I dont understand why Nedbank cant change a cell number. I've changed it 10 times in the last 2 years, I've even had it deleted from my account & still the OTP's are being sent to a number I discontinued 2,5 years ago! I'm so over this nonsense. The Consultants keep telling me they changed it but when I try to add a recipient or buy something online the OTP is sent to old no. WHY?