At my wit's end!!

I bought a phone at Edgars on the MTN network in 2012 - this was then insured. I bought a new phone and was no longer using that one, but continued to pay insurance on it. This was about 18 months ago. I looked for the phone earlier this year to use it, only to discover that I could not find it. I then contacted MTN to blacklist the phone so that I could claim on the insurance – but have had endless issues with this. They are now telling me that because I have not used the phone in so long, and also because I ported my number to Vodacom, they cannot help me. They cannot blacklist the phone, and I therefore cannot claim. I think it is ridiculous that I have been paying all this time, and now I cannot lay claim to this as MTN are giving me the run around. I've been in and out of the Gardens centre MTN shop (completely unhelpful), called the "helpline" as well(equally unhelpful). Please could you advise in this matter. I am extremely frustrated with this process, and it seems I am being punished because I left the network. If I do not receive satisfactory results at this point I will be forced to take the matter to the Ombudsman.