Vodacom wrongfully handed my mother over for Dept collection and refuse to assist in getting it sorted

Vodacom handed my mother over for an account she closed over a year ago. Today she is unable to get a home loan that she desperately needs because of an account she fully paid. My mother has had a beautiful credit history until now. When looking for assistance from Vodacom they keep telling us to send emails and call the people that they handed her over to. They are not willing to assist at all. She sent an email months ago and they promised to investigate and they never did. Now mother is frustrated and left helpless. How does she lose a home she loves over an account she paid. I need urgent help.