No resolution

I was sent a R500 Pick pay voucher after purchasing airtime. I went to shop at pnp and was told the voucher is not valid. They call mtn for 9 days cause I went every day and no resolution. After several attempts the customer service agent said, you only get a vouch if you bought a cellphone. So my voucher is not valid because I bought airtime. Even if it was sent in error the voucher must still be valid. So unhappy with this