FNB "dispute" assistance

I am trying for several weeks to dispute a debit order but cannot do it on the banking app if it is above a certain amount. Many times I was given a number (9 to be exact!) on the "secure chat" (which I can forward to you) to dial for assistance. My only assistance every time is a computer voice telling you to use the banking app for disputes!!! All other landlines numbers you phone - "the number you have dialled does not exist"