Rejects phone claim

I have a phone contract with MTN , I took the insurance with them which happened to be Santam. I lost my phone and my claim was rejected because I was using a different sim card in the phone, which I mentioned upon purchase that i won't be using the MTN sim card and it was said I can use my old sim. Now that I'm claiming they don't wanna replace my lost phone. Why have I been paying with no fail if I would put in a claim and it will be rejected?