Tracing cell numbers and debt collection

Got a call from debt collection agency CFS on cell about FNB credit card - I never had one. They clearly traced my number and tried to match it to the right person because as soon as i said i dont have a FNB account they hung up. I called the number and got a call centre for CFS debt collection but if you get through to reception, they say Brooks and Luyt attorneys. They first refused to say where they obtained the number and then when pressured claimed it was given to them by FNB - which should not be the case. Turns out they are looking for someone with a similar name. Now - I would like to know please, where did you obtain my number ? I asked that someone call me back as this could mean my sim card got cloned or i will get blacklisted for someone else's debt. No response. Surely this is not good debt collecting practice?