Inaccessible Call Centre

I've been experiencing problems with Vodacom . For the past months , every time I recharge with R25 airtime or more, R10 rand airtime disappears. I''ve continuously tried contacting the Vodacom Call Centre to find out what exactly is happening , but each time I call they tell me that the line is busy and I need to call later. For how long do I need to contend with losing airtime with no explanation. If it's not the data that depletes rapidly, it's the airtime that disappears . I'm very frustrated and annoyed.