I've had it happen to me a quite a lot recently on my mobile. If I don't recognise a number (and I'm in front of my PC), I'll just do a search for it, and it tells me who it's off most of the time. (Or just go on whocallsme.com)
But the biggest thing I've found out is that there is a lot of money to be made these days just by companies selling your information. So you could innocently apply for loans/credit cards etc, and before you know it, you've got all sorts calling/texting/emailing you selling all kinds of shite.
And don't even think that answering your phone and abusing them will make them go away. All it proves to them is that your number is 'active', so your number will again be sold on to some fucker else so they can mither you even more. Same with SMS spam. Don't send 'STOP' to stop the SMS spam. Again, it just shows that your number is active, and you're bombarded with text messages all fucking day long.
Hope this has been some help.