My thanks to Anne Irving of Irvings Solicitors who recently reported this. Someone had visited their website and filled in an enquiry form. The phone number they used in the form was a Premium Rate phone number. When the member of staff called the number they got an ansaphone at the other end, with a message saying we are connecting you, ringing but not being answered. Now, bear in mind that the form has what's called a "captcha code" on it to prevent it being filled in by a computer which means someone did this manually.
Something to keep an eye out for on your site.