Extremely unorganized and unprofessional

I am very frustrated and disappointed at the service that Vox failed to provide. When I moved houses in May I cancelled my month-to-month contract with Vox and received a confirmation stating that no fees were outstanding or applicable. It was only months later, when the service had been suspended for a while, that I received an invoice asking me to pay 2500 Rand. When I did not do this, as I was communicating with Vox numerous times about the fact that the service had been suspended and I was therefore not going to pay, my case was handed over to Debt-In who very have very unprofessionally called me every day for about a month and a half. Instead of introducing themselves first, they immediately request me to give them my passport number without mentioning who they were, what company they were calling from and what the call was in regards of. After sending multiple emails including the paperwork to Vox and Debt-In, I am still being harassed telephonically on a daily basis. My latest experience was shocking for the lady one the phone was incredibly rude, yelling at me through the phone and not letting me finish my sentences. She was asking inappropriate questions regarding my financial situation and dismissed what I had discussed with her colleagues before. If this continues throughout next week, I see myself having no choice but to take legal action myself.