Unsolicited calls - Embarrassing

As a teacher, I am unable to answer phone calls during school hours. I ignored a call 4 times from a call centre. Not long thereafter, the deputy principal calls me to attend to an urgent call in the office. The caller announced himself to the receptionist as “Nedbank Credit Card division”. I answered the call and this agent is offering me a credit card!! The ludicrousy of creating the implicit impression with my employer nevertheless, that the call is regarding debt collection. I have NO accounts with Nedbank, nor do I now intend to have any! Therefore, Nedbank obtained my info elsewhere. At this stage, I require full details of where my info was obtained from, along with the proof of permission to contact me.