I want my phone back so that I can settle the acc

I've bought a phone and it had lcd problem then I took it in for repair Shuab mtn qualified sales man told me to not hand it in because mtn is over charging, so I asked him what must I do, he said I must take it to the small phone shop, then I asked him about the warranty of the phone, he said I mustn't worry the warranty won't fall away. Then I took it in to the small shop, a month later my phone was due for software update, I took it in, mtn told me that they not going to help me because the phone it has been opened by someone else, since that day I've never use their sim card and I've never receive my phone back because a store manager told me that she will make a plan, until today I don't have my phone back, now my credit record is bad, help me to get my phone back then I'll pay the account<br> Regards<br> Mr S Dyifolo