Dave - that's weird, and the first comment on this site that suggests the company are truly reportable. I would email them about this. It's one thing to be a company calling to try and steal business, and another to be a company making unnerving prank calls. If I'd got a call from a number I didn't recognise saying I'd placed a personal services ad in the newspaper - and hadn't had the presence of mind to google the number and find this site - I'd be absolutely terrified.
Also, I called the police and reported the incident, and got a crime reference number which means that if they keep calling and I end up needing to change my mobile phone number, I can quote the reference to my service provider and they'll send me a new sim for free. This is worth doing if the calls are upsetting anyone.