One account , two companies

I paid up my University of Llimpopo account fees 10 years ago,even graduated and got my Degree, however Debt Tracker Collection keeps on sending messages and calling me. Apparently Debt Tracker claims that i owe them because the account was handed over to their legal department while on the other hand the University accepted my money. Who do i owe here? I called the University but they keep promising that it will get sorted. I am unhappy and confused with these two companies.