Always Bad service

I bought iPhone 6s on contract. Last month it got lost n insurance gave me a new fone. But the fone started giving me problems just aftr I Cancelled my insurance due to bad service I received while claiming. Now I took my fone to mtn for assistance cus Dat fone doesn't have 3month n ur consultant at modimolle store are refusing to assist me,they say they want proof of purchase, but which proof cus it's on contract n m still paying it? N they want proof from insurance that state why they changed fone color cus it was white n they gave me different colour, n I didn't ask when they gave me fone,and insurance is also refusing to give me that proof why they changed my phone. So what should I do with this new fone Dat is not working while m still paying it? Please assist