Cancellation of contract IMPOSSIBLE

Due to non existent reception at my home and work, Cell C came out and confirmed lack of signal and allowed me to cancel my contract with no penalties provided i hand back the handset. I handed back the handset over 3 months ago, and they refuse to cancel the contract until they received the handset at their warehouse. Cell C's inability to find the phone within their own system has caused me to pay full contract value for a phone I do not have anymore for over 3 months. I have contacted them innumerous times with absolutely no success or meaningful assistance.