Cannot change OTP cell number

I phoned the call centre to change the cell number linked to my account and I was told that this was complete. When trying to buy online, I never received the OTP and when I checked my online profile, the cellphone number was not changed for the OTP. I was then advised that I can go to any ATM and change the number there, which I had done and saw confirmation on the ATM screen that the number was changed. Again I have tried to buy online and never recieved the OTP. When I checked my online profile, it again shows that the cellphone number has not been changed. This is extremely frustrating to have to go back and forth so many times just to have a cellphone number changed. The reason I keep the service with FNB is because it is meant to be more convenient than having to queue at a branch but it defeats the purpose if I have to run around so much and then will still end up having to go in to a branch and have it done anyway. I am very disappointed.