Bad customer service

I took a contract with Vodacom in March last year. The contract was done over the phone. The phone is used by my son and I never checked the statements until this year in April. The reason is that the statements are delivered at my residence and I was working away from home. On the statement I saw that there is a number on which airtime was loaded every month which I was not aware of, since I was never given a sim card, only received the device. I went to the Vodacom shop to enquire, and I was told that I can do a sim swap so i can be able to use the airtime. The balance of the airtime was over R700. I did not do the sims swap immediately, I only managed to do it on Monday, 28 August 2017. When I checked the balance it was R660. I phone the Vodashop and I was told that I must phone 082114 or 082111. I have been trying to call since Monday everytime I get a message that they have a high calls volume and I must leave my number and a consultant will get back to me, I am still waiting. I even wrote on the Vodacom Facebook page to let them know that I cannot get through to them and I have a query and still I have not received any response. I want Vodacom to explain what happened to the other airtime since I never used any and have been paying every month since march last year. How did they come to the R660 that was loaded on my phone.