Unable to unblacklist phone

Hi There<br> I bought a iPhone 5s from a guy called John on Gumtree, I used to phone for a couple of months now with a Vodacom sim Card, however last week when I tried to put in a MTN sim its not registering the sim, went to Vodacom they tell me its blacklisted (Yes its blacklisted but still working on the network that blacklisted it), I signed the unblacklist form. Now I am told that they wont be able to unblacklist the phone cause I am sharing the same surname (yes apparently that's not allowed) as the original owner which is now Frieda Van Der Merwe and not the John guy I bought it from, it seems she still has money due to Vodacom, why must I pay for someone that is not paying their contracts, there is legal channels to follow to get your money back, I paid for the phone that is working 100% using a Vodacom sim but doesn't work with any other network, how is that even possible. I am sure you could claim your lost money back via insurance. The fact that Frieda didn't pay her account should not be my problem. We live in South Africa, people buy and sell phone from multiple providers. I want my phone sorted, since I paid for it. You sort your issues out with the account holder.