empty promises

I took two Vodafone tab 3G on contract for my kids, I was told that I can make a call using Vodafone tab to my surprise one can make a call and other one can't so I went back to the shop to see if they can help me or maybe they can replace the faulty one , they told me that all the can do is to book it in ,I told them to book the tab in . they book it for about four weeks wen it was brought it was still the same problem and they even refuse to exchange it . I am not happy coz the tab is not working as they promised me and they refuse to help me and I have been vodacom s customer for longtime now the don't even care about my problem