I've been buying MTN airtime for 3rd time now, whenever i load my airtime i don't get what i bought

I've been buying MTN airtime for 3rd time now, whenever i load my airtime i don't get what i bought.i get way lesser airtime and i can not get hold of their call centre,i bought airtime for R12 and i got R1 it's happening for 3rd time now.please assist