Telkom billings dept.

Please tell me what I can do with Telkom? After visiting 3 of their stores over a number of months, calling them at least 7 times in the last 2 weeks and sending the same email to them, after being told to do so. The emails bounce back saying their mailbox is full, therefore they're not reading anyone's email and I cannot get any answers on the phone, that is if they don't cut the call before hand after asking for all my details but I never get a call back. They tell me I owe them money but I haven't been credited from my last payment so this problem has to be sorted by the billing department. It is beyond terrible to have to waste so much time on something and never get a call back or a decent response to anything. I'm at my wits end!