BAD SERVICE AND WRONG PHONE SENT

I have been with MTN for over 6 years - my upgrade was due in May 2015 had no response's from the store and have tried on many occasions only to find they were on strike. eventually after many messages the store called to upgrade my phone. they offered me the galaxy A3 and send the galaxy pocket phone, I am half blind so what good is that small phone when I have a samsung mini which I have to use a magnifying glass. the courier guy first made me sign the docs then opened the box and I saw the small phone he could not take it back and said I should deal with the store..now how and who cause I have been given the run around and have over seven numbers to call but cannot get any joy. I just want the phone they offered and cannot get hold of anyone..please assist me,. I do not want to be stuck with a pocket phone for another 24 months