Disappearance of Credit Balance

In November / December last year I purchased a sim card for the purpose of travelling abroad so as to have control over my spend whilst in the UK. I had in the region of R800-900 credit on the sim on my return to SA in January. On purchasing the sim I mentioned to the sales person the reason for the purchase and that it would be used exclusively for my overseas travel. On inserting the sim into my phone in readiness for another trip I discover that the number is no longer registered on the MTN network and the Credit Balance has disappeared. I approached Zair from Gateway in this regard and who in turn addressed a mail to the person he thought could help me. I was told that I could not have a copy of the mail or the contact details of the person he sent it to. I am still awaiting the courtesy of a response from both Zair and whomever it is at MTN who is supposedly handling my Query. My question is, how can they just on a whim decide to recyle my number and do what ever it is that have done with my credit balance. I eagerly await a response on this matter.