SAME FAULT RE-ACURRING

I purchased a Samsung Core 2 on the 4th March 2015 and I have called Samsung on various ocassions and taken my phone into the service center in Boksburg 6 times for the same problem whereby my phone displays a different number from the one that I am calling or receiving a call from. It has had 2 software upgrades, was told it was a sim problem to call MTN and ask them to change some settings which i did, then was told after that to do a sim swop - i did. then a new a new lcd was put in, then a new motherboard and it is still doing the same thing. I am getting no joy. I called Samsung on thursday and was told i must wait 24 - 48 working hours which is up at 3 this afternoon and still no joy.