The store doesn't want to asses the phone due to dents found "inside" the phone.

I have a contract with mtn I purchased two iPhone 7. The one I was using was heating up, up to a point where I can't even hold it, and this happened in few weeks from the day I bought it. I took it back in store and they book it in for repairs. After two weeks they told me warranty assessment was rejected due to dent they found on my phone. I went in store to see which dent they were referring to, both the store manager and I could not see any dents instead she told me that she was told that the dent were found inside the phone. They told me that they will send my details to the area manager he will get back to me. It been two weeks now and I haven't heard from the area manager I call them almost everyday and they keep telling me one and the same thing that they did send my details to him and they will send it again...I don't know what to do now, cause I'm paying the store for something I hardly used.