Frustrated

I was handed over to Sanceda, i made payments as agreed, Last payment 2015 Nov, i went to make another payment teller informed me that the account is no longer valid I assumed im paid up. I get a call from marang who say i still oweR6372.27 I ask the consultant that how is it possible they get hold of me on my new number and no one could get hold of me on my old number, it changed last year when my phone was ******. I am unemployed due to illness i informed Bianca of this, n now they want me to commit to a payment arrangement i said i cant commit to what they asking me to pay, as i have no financed. I am frustrated cause I paid my account and no one informed me that i still owe, i left messages at Sanceda for someone to call me no one did, 2 years later i owe so much money for interest occurred, I was not at fault i paid my account and I assumed I was done when thr bank didn't accept my payment due to the account for the plaintiff didn't exist anymore. Why didn't they contact me to inform me, why wait 3 years to let me know i owe them under a new company. What can i do, i need assistance regarding this matter as i sm unemployed at this time.