ICASA, tell them the phone is illegally locked.
Bought a Samsung Android smartphone on prepaid in Jan 2013. Not locked.I think those prepaid phones are usually network locked, especially the ones you can buy from places like Jet Stores, Edgars etc.
In the last 8 years or more none of my MTN phones that I got from MTN dealers were locked to their network. I should probably also add that all my phones are on contract and not prepaid.
There was a thread about network locking a while back, the legislation was drafted but never implemented. So in short, it's not illegal. Seems quite unusual though.
ICASA also moved to prohibit the practice of SIM-locking, handset-locking, software-locking and any other type of locking a handset.