Billed and still no refund

I was overcharged for my 3G in March and queried it. On 30 March I received a sms stating I will be refunded. On 30 April I called again and was told I will be contacted within 3days. On Tuesday I contacted Cell C again and was told I will be called in an hour. Its Friday and no call yet. What must I do to get my money back? I'm just glad I'm cancelling all my contracts with Cell C. What's worse is the way the call centre agents speak to you as an upset client. Clearly Cell C doesn't train their staff properly. Bad service is an understatement.