Need Urgent Assistance

I need assistance with my contract. I was due for an upgrade in March this year and received a call telling me that a company called Summer Sun has couried a phone to me, I told them they should return it since I didnt want a phone yet. Now when I go to MTN to upgrade Im told I cant because of that company. I dont understand why MTN has given my details to another company without my consent. Now Im having trouble trying to take myself out from that company, I keep wasting airtime calling them with no assistance. When getting the contract I never approached any Summer Sun, I approached MTN. So please help me in this regard, Im very tired and I need assistance urgently!!