makanda finance threatening & harassing me at work

I am now fed up with the consultants at makanda finance, i have more than 30 emails requesting them not to phone me at my place of employment. we are not allowed personal calls. they keep on phoning me at work and threatening me that if i dont pay he is just going to keep phoning me at work. also leaving messages at my co workers. if i request them to phone on my cell they refuse and if i hung up they call back to my work. threatening me that if n pay they will stop phoning me at work? they dont want to accept my payment offer and phone me at work numerous times a day? i am sure this is against the law? they refuse to hung up if i inform them that i am in a call centre and cant discuss my personal stuff with them as i am also not allowed personal calls at work......then they just phone right back to my place of employment?????.??