Breach of contract

I had a 2 yr cellphone contract and they were deducting money from account. They stopped after 2 yrs as l finished the term, 1 yr down the line they called to say l still owe and l asked why bcoz in 2012 was my first yr of contract and after 2 yrs u stopped all by yourself and by the time my phone was lost you blocked your airtime and dropped installment to R99 but they insisted l must pay. They are calling me now and again saying l must pay R388.00 and l am telling them l will not be able to. I feel that they are now becoming ********** and they say they are not going to stop calling and they will continue adding charges.