My phone went for repairs never came back

I bought a phone at PEP Cell in Pretoria Station Square shopping center, it was fault within two months. They took it for repairs, they promised to give me a call after six weeks until today they have not. When they took my phone for repairs they did not give me a invoice. Now when I inquire about my phone they ask for invoice. I need help to get my phone back. Even if try to give them my ID book they said they can't trace my phone but the time I was buying the phone they asked for it. My number is ********** 227