Repairs to phone

I have been to THE worst Vodacom shop. Clearwater Mall. My son handed in his Sony m4 at Clearwater Mall as I was abroad at the time. He handed it in on the 1st February and was told that they do not give receipts for the phone as it is booked in on the computer system and he would get an sms. He did not get one. He went back on the 16th February and after searching for some time they found it in the store, it was not sent for repairs. They phoned him on the17th to say there is costs to repair the phone although it is under warranty. I went to the shop on the 21st February and was told that the phone is still at the repairer and they would find out why there is a cost and phone me back today. I received a call this morning informing me that there was a software problem that they fixed, but there was an outstanding amount. When I got there I was informed that the phone had NOT been repaired and that they would not honour the manufacturer's warranty as we had used an MTN sim in the phone. I have never in all the years been told that I would loose my warranty if I use another network!!! Why tell me this after three weeks? Is it Company Policy to lie to customers or a branch policy?