MTN bait you with vouchers and once the contract is signed, retracts.

MTN sent me a WhatsApp which I will copy below. Right after I clicked on the "Pre-order now" button, a sales agent contacted me. I specifically remembered mentioning the voucher after which we concluded the contract. I received the phone and was told I will receive the voucher after the 10 day cooling off period which was fair. After the cooling off period, I enquired again and was told that I will not get the voucher. Apparently they listened to the recording and no voucher was mentioned. I am now going to ask for the recording. But this is very bad service as the sales agent would not have had my details to call me if I did not respond to the pre-order promotion and I would not have responded to the pre-order if it was not for the voucher.