Silent calls and spam targets cellphone users

I have two mobile phones. One is purely for personal calls and so that number is only known to my family and closest friends. The other is purely for business calls, the number is printed on my business. A couple of times last week I received what are known as silent calls: telephone rings, you answer, nobody speaks and the line drops after a brief whirring and clicking. It used to be the case that most silent calls could be put down to embarrassed types who had mis-dialled. These days they are much more likely to be a precursor to spam.