Bad Service

I renewed my contract with MTN on the 18 July 2015 and received my new Samsung S6. I took the phone back to MTN towards the end of October after numerous people complained they could not hear me on the phone. I was told by MTN to take the phone to the Samsung Smart Center for Repairs which I dualy did. That was the last I say of my new SAMSUNG S6. I have been phoning Samsung to get my phone back but I have no luck and keep being told phone is waiting spares. I went back to MTN to assist me as I have my contract through them and was told they cannot help and even they I pay subscription that is not their problem , I must sort it out, but still I must pay my monthly subscription. What kind of service is this. My contract is with MTN not Samsung yet I am supposed to just wait, for how long. This is totally unacceptable and would like some one on a senior level to either get my phone repaired or replace the phone. If not either one can be done I want the contract cancelled with no penalties