Incorrect Handset provided after Repairs

On 06 March 2015, I received a hand set which was previously taken-in for repairs cracked Screen. On delivery I informed the courier people that they delivered a wrong handset and I was told that I must take up with MTN as they are just the delivery people.On Monday the 9th March 2015, I called MTN and a call centre agent answered the phone and he introduced himself as a Mr Patrick Mfelang, I explained what happen on my phone and gave him the handset serial number:*** and he re-iterated that those courier people delivered the correct hand set, I informed him that the serial number is correct but the hand set that I was using before was a dual sim card and the new handset is the single sim card one. I informed them that I have proof and a box of the hand set to show that the hand set that I have been using before was for dual sim card, the handset that I am using now is totally different from the one that I was using previously. I complained to MTN repairs with this *** and on MTN Complaints Department with *** and without any assistance. I was sent from pillar to post. No one can explain what really happened to my original hand set, which I want.