Shocking service

My father was financed by Nedbank in 2013 he bought a vehicle through them and the vehicle was paid up in 2017 July, he sent a proof of payment by an email and they promised to send the vehicle certificate through the mail, the certificate was never sent, he tried calling their call centre but unfortunately he happened to be attended by a rude lady that he could nt stand her attitude and decided to drop the phone during conversation, my father passed away last year November, then in December I decided to send an email to their car division explaining the matter but up until today I have nt got any response from them, my concern is that why are they keeping this certificate and what must I do to get the vehicle certificate because the vehicle was paid up 3 years ago.