Rude employee

I purchased two cellphones from the cna in kolonnade shopping centre and after a week they stopped working and I tried to return them this past saturday and I was assisted by the most rudest employee called frank,he was insulting and shouted at me infront of customers it got to a point where he was even threatening me with violence. I called his manager to find someone else who could assist me but his attitude went from bad to worse,and he kept on grinning and making harsh remarks and now two days later