They have contracts with their phone companies that allow them to call you. Your contract does not override theirs.
Contract Law 101: A has contract with B. B has Contract with C. A and B's contract in no way binds C. A telemarketer's contract with a phone company in no way effects me, and vice-versa.
Contract Law 102: You cannot contract to do something illegal. If a telemarketer enters into a contract with a phone company for services, the telemarketer cannot use those services for something illegal and hide behind the contract. So, a telemarketer may have a contract for "unlimited phone services" but those services are limited to legal behavior.