50% Discount advertised not honored

I contacted DSTV regarding the sms received in December , in the sms they advised that if you pay before the 31st you will qualify for a 50% discount in January, I paid my account on the 30th of December. When I inquired about the amount due for January I was told about the full amount in which I disputed, the agent that was assisting me told me to hold on while she is checking with accounts depart. I was on the line for half an hour listening to the music, she never came back to me until I hanged up. The level of service from the call center agents is really bad.