Misleading Contract Sold

About 3 years ago I took up a mordem with MTN for R149.00 per month. When I asked for the mordem I was specific to the Sales lady at MTN Umhlanga branch that I wanted to pay nothing more than R150.00 per month. I have used it for sometime now paying it in good time via debit order and I understand the fact that I must pay my creditors on time as I work for the Bank and we use the same concept in order to approve or decline my credit. My nephew and my brother-in-law would ask for my laptop with mordem to do school projects and job search and I would give to them as there is no clause on the T&C's (like as in the Bank's pin numbers) that you cannot give your computer to family to use. Obviously they used these tools to do more that view internet and search for jobs, it seems they downloaded music as well. My complaint is with the fact that I was always confident that my monthly instalments was only R150.00, no one ever advised me that its my responsibility to have it capped at R150.00, when I went to MTN they showed me a contract with my signature, but as per CPA I am to be protected as a client and MTN is exploiting me. I now have these MBD attorneys calling and texting me