How come Cell-C selling your personal info to other people?

How come Cell-C selling your personal info to other people? I am receiving calls from UNKNOWN callers to say ,you got Cell C contracts and would like to provide additional services,please provide your email id and cell number to proceed? Being a Customer ,don't you have rights to keep your personal info secure by the service providers? how come all these people are selling your personal info to other companies and you will get Nuisance calls everyday....