Cellphone Repairs and Warranty

It came to my surprise that I bought the MTN cellphone 11 months ago. Now the cellphone can't switch ON, it dead. Now I'm being told that my warranty is void because I did not use MTN network. Why I was not told that information when I bought the phone? Why must I use MTN network if the signal is not good in my area? I was buying a cellphone not a sim card, MTN is terrible and I will never and I will advice others to stay away from MTN cellphones because they have ******* after sales service.