pay back the money

I had a contract with cellc with i had taken out in 2015. Due to cellc me debiting me all incorrect amounts, i agreed to settle the account. After agreeing to settle the account i found that i had a credit owed to me of R500. After calling for numerous times and sending them emails, i have still not received my over paid amount of R500. why is that when a contract is not paid on time, the call centre keeps phoning and hounding to make payment but when we are owed nobody is even courteous to phone or give me back my money. its been such a long time and nobody called me back- they have taken my banking details about 6 months ago but still havent given me a credit. i somehow think that this ******* business ethics.