please assist on a phone matter, don't know anymor

bought a sony c3 around March last year and started using it in June after a month it started acting up(switching itself off) (sometimes when it rings doesnt want to answer or touch) but at the time was in Pretoria so i come back and took it to MTN then told me i should send it to the shop i bought it for repairs since its still under warranty and i took it where it was bought which is Truworths Riverside Mall which when i got there they gave me attitude and asked me why don't i take it to MTN and started acting on me asking me wheres my warranty slip since im only carrying the phone slip and i told them i don't know where it is and the lady named Nozipho told me they can't help me so i should bring the slip then there they can help me, i didn't argue with them as they refused to ask for the manager then asked someone to help me about the issue which told me to report to hello peter and i did and after less than an hour Truworths Riverside called asking me to bring the phone, i handed it. they called today which is the 3rd time sending it to repairs finding out again it still has the same problem while was still at the shop and its been while since January i don't have a phone!!!