Vox Telecom unable to transfer number

I have a sip account with Vox. My number was originally ported from telkom. I now wish to cancel my account with vox and transfer my number to someone else. Vox is now telling me this is not possible. The number would have to be transferred to another isp and only then can it be transfer to another client on vox. Vox really knows how to make there clients lives difficult.