Uncolicited nuisance calls

<p>For the past three weeks I have daily been receiving numerous silent calls from a number ( ********** -9) which belongs to Neotel. I have never had any dealings with them so I have no idea why they would be calling my unlisted landline number every half hour. I phoned one of these numbers back from another phone, but it is just a machine which doesn't identify the owner of the number. Conclusion: SPAM. How can I make them stop these incessant calls?</p>