The worst service provider

So I’ve got a contract with Telkom which is R479pm and today they send me an SMS saying that I owe R681.70 and I don’t even use my Telkom SIM card so what more are use charging me for? I had tried so many times contacting but we don’t get helped this is the worst service ever! and they do not get back to you at all . Why am I being billed more than what my contract states when I not even using the SIM card!