MTN South Africa

I bought a cellphone at Jet store in Menyln Mall around November 2015, and I have never use the cellphone because it need a Sim lock code even when I insert vodacom Sim card. The problem is with a phone. I tried to take to MTN shop, but I was told to take it to Jet where I bought it, then I was I told I created the sim lock code I was surprise... I feel like to change MTN network cause the service they rendered to me was very, very, very poor!!!! it doesn't show that I am a valuable client since I do have two MTN contract.