MTN Cell contracts

We are with MTN for 30 years now and were always satisfied with their service. We are on contract with them. January/February this year someone from MTN called me and said I am due for a new phone and can go to the nearest store which is Robertson. The guy working there gave me the new phone. Soon afterwards I got blocked on my phone by MTN. Seemed as if I were ****med and I owe MTN R11 000. They just do not listen to me and says I am in the wrong. I get nowhere with them