My number is given to someone else

A few days ago I received a message saying my personal details have been changed, when I look at the details they belong to someone else. I tried calling customer service but the system doesn't allow to speak to an agent, I then went to the website to speak to an agent and to my surprise there's no option for that, instead you request a call back. I did that but till today I haven't received that call, now I'm receiving calls from people I don't know. I don't understand how is my active number given to someone else.