Handed over to attorney after account was paid up

I paid up my Vodacom account in January 2015. I received a paid up letter from Vodacom that confirm my account was paid in full. On 16 March 2015 I received a sms from VVM (a specialist in debt recovery) that Vodacom handed my account over to them to recover my debt. I send to Vodacom and VVM my paid up letter as proof that my account is paid up. Vodacom confirmed this and said they will ensure all their systems will be updated accordingly. On 11 April 2015 I received again a sms from VVM regarding my Vodacom account. I phoned VVM and explained the above to them, but they claim Vodacom instructed them to recover my debt. When I contacted Vodacom, they only said they are not responsible. I have again emailed my paid up letter to VVM, but without any success. Vodacom doesn't assist me, they only send me my paid up letter each time. And VVM keeps harassing me regarding my outstanding Vodacom account.