Phone locked on network

I've been with Vodacom for 3 years now. On my previous contract with Vodacom I had a Samsung Galaxy S3. I now have a Sony Xperia on my new contract.<br> <br> I want to give my old Samsung Galaxy S3 to someone. But they are on Cell C & the phone do not want to except the sim card. <br> <br> I phoned the call centre and spoke to Andiswa who told me that sometimes their phones are locked on a network. She could not advise me how to unlock it or how they get permission to do this?<br> <br> I have paid for my phone for 24 months. It's mine now. If I want to use another network in it, I should be able to. And it is against the law to lock phones in South Africa.