arrears account dispute

i had a contract with vodacom and was unable to pay, vodacom contacted me in this regard and we came to the agreement that they will cancel my contract but i will still be liable for the full amount but will be able to pay it of in smaller installments. this happend and i paid all that i owed in full. after i settled the account i phoned vodacom to confirmed that i owed they nothing. they in turn rep**** yes the account is settled. after a couple of months i received a sms stating the they are handing me over to legal due to lack of payment. i contacted the legal department and started the process to resolve this issue after a couple of calls and emails the legal dept. requested my statements from vodacom to compare with the statements that i had and the comparison just confirmed that i owed vodacom nothing. the legal dept said i need not worry they will sort it out. again after a couple of months i received a sms from vodacom stating that i owed them money. i have tried to resolve this issue and have just hit brick walls. vodacom just keeps on referring me to different dept. and no one can help me. i am at the point were i am going to take legal action against vodacom for *****.