Poor customer service

I recently got a mtn contract with Mtn. I signed the cellphone contract on the 15th of June and I called on the 14th of July to ensure the phone. To my surprise when I got the July/August invoice the insurance premium was not included so I called the insurance I was told that they couldn't find the recording as I had called via the customer care linr"135" and I was transfered to the insurance line. I am so upset because I insured the phone with the 30days but for some reason my sister's phone was not insured and they are making it my fault that they can't find the recording. In other words when they say the recording will be available at request is a lie. My sister's phone should be insured as per request because I called in on time which was within the 30 days.