Overcharged... AGAIN (going on 8 months now)

On 04 September 2018 I upgraded my account. The monthly instalment was quoted as R 1 639.42 which I accepted. This included a handset fee of R 586.50 (I have a copy of the quote for reference).

On 16 October 2019 I was offered an upgrade as a premium client. As per normal, I got a quote for my new monthly payment which would total R 3 088 for an unlimited line for my business with a new handset. I accepted this new monthly instalment quoted (I have a copy of the quote for reference).

However, I have since been charged an additional R 586 per month from October (going on 8 months now) for my old handset bringing my total monthly payment up far higher than what I accepted or what I was quoted. No mention was made/quoted/specified that my old handset was excluded from the quote and that it would be charged extra. Had this been specified, I would NOT have accepted the quotation.

I have tried to get clarification from Customer services and have requested feedback on numerous occasions. Do I need to go to the Ombudsman or a lawyer to get this fixed? This seems completely ridiculous. I accepted a quotation (which was very detailed). You cannot now change the specs of my quote or account after the fact.

I look forward to your most urgent reply.