Over charged again

Since the signal in my area is atrocious a 20% discount on the subscription of my data and voice contract was instituted years ago, with the understanding that it will remain in place until a tower is built and the drive tests show an improvement in signal. I raised this issue 2 months ago and was assured that it would be rectified. Upon receiving my latest statements I see that the discount has been app**** to the data contract but not to my voice contract. I'm not sure how many times I have to raise this issue before this matter will be corrected. Please note that the agreement is both legal and binding and if it is not instituted with regard to my voice contract with immediate effect I will resort to legal action and hand this matter over to the ombudsman