FNB - Charging incorrect interest rates!!!

I have never had any issues with FNB until now and the current matter is infuriating me. I contacted FNB on the 15th of July (***-10) to inform the team that I am being charged an interest rate of 28.25% instead of the new agreed rate of 13.25%. Its been more than two weeks and nobody has responded despite me calling and following up. Today I called once again and the person assisting me told me I was quoted incorrectly. What does this mean? I only accepted the agreement because the quote was acceptable. I will not pay more than that. I have the recording of the call where I agreed to the rate on the quote given. I was given the new credit agreement more that 2 months ago. Furthermore the call assistant put me on hold today without telling me to hold on. Eventually she hung up the call after me waiting 15 minutes. This behavior and treatment is unacceptable considering I am a client now for almost 10 years. Extremely disappointed in FNB especially considering all the people I have converted over the years. Can somebody PLEASE HELP ME?