Samsung Galaxy S5 Blacklisted/Blocked

Hi guys , picked up a S5 at a train station , Tried finding the owner for 2 days but had no luck , taking the phone to a police station in my country would mean that some corrupt guys cousin is going to get a new phone . Now it had a CellC sim card in , wiped the phone and tried my MTN sim card it won't take it says it's not registered on the network BUT if I use my telkom mobile data contract sim card in the phone it works fine without any blocks or anything . So logic tells me it's been blocked on some of my local carrier networks or "Blacklisted" as such . Please help , I would like to use my MTN sim card in the device