Bad service / Bad treatment and messing up my credit record

Had a loan with them missed a payment due to job loss,,after I got employed again,,they deducted 3 * 1775 installments, called to complain that I cannot afford that, they told me they can only reverse one payment and the only option is to dispute the other installment and I did that,,,which means they got one installment as they should. Then I was called after few days to be offered a consolidation as I was now in areas,,I took it and started paying off the amount and have now finished it. Now an amount of 2044.93 is still due and has been handed over,,,when I called to get my balance on the consolidation, they told me that the consolidation left that one installment by mistake...now I need to pay it. I have been sent to pillar to cost about this and spoke to more than 15 people and no one can make sense of what happened and how it happened and who's fault it is. They all say we will call you back which never happens its been 2 months now and this is affecting my credit record.