Contract ended in 2016 and number was converted to pre-paid and now in 2018 they are asking that I owe them money

I had a contract with vodacom which ended in 2016. I asked vodacom to convert my number to prepaid. Instead of converting they recycled my number ********** . It took longtime to get my number back on prepaid (i sent numerous emails in 2016 and visited Vodacom ****arney Shopping Mall several times but in vain. Then I posted on their facebook page and then I got my number back in 2016). Now In 2018 I am being threatened by Hammond Pole Attorney that I owe money under ACCOUNT NUMBER: I ********** . I answered their email but they are not responding to my email but are calling me everyday. I sent them several emails but they are not responding to my emails but calling me everyday. What must i do?