My number given to another person

My name is Vela Vincent Huza and i have a contract with mtn which i have been renewing every time when that time comes, earlier this year i called them and informed them that i will not be keeping the number mentioned below for sometime this year due to personal reasons but i told them that the measure is temporal because i will keep the number active again towards the end of this year. They said that will not be a problem and they kept on deducting from my account as usual but then i was shocked this week when one of my friends told me that somebody else is answering when the number is dialed. I thought he was joking until i found out myself,how is that possible?I thought i am the only one to use that line as i am paying for it that is why i think i am being wronged here and i deserve an explanation, i've sent them e-mails but they are not responding.