Defamation of character by FINCHOICE

Been a client for years and years, not once have I made a late payment... In Nov last year I called them in connection with an sms confirming my "change of phone number". About 3 times in a row there were calls to Finchoice regarding this and I was assured it was resolved. I later checked my account and I had more than enough readily available for use but the request was declined sighting that I should pay my creditors and them or I should pay 'in time'. Taken aback, I called and I was told the "availability" of funds is determined by Credit bureau linked to them, I must call a so called KWIKSERVE. I checked the Credit Bureau and Experian, I even checked CLEARSCORE and there was no bad record. I emailed finchoice and asked them to furnish me with details of this person or company or whatever that's determining my financial fitness, until today. Kwikserve number is FINCHOICE number and I've tried calling several times and they no longer pick up my calls... I guess they now Know my number. I am willing to take this as far as it can go even if it means I go the legal route for defamation of character lawsuit. Finchoice branded me a bad or late payer and they tell clients lies about them attending to their queries. This is disgusting, unprofessional and an insult to honest clients like me