How am I being crank called like this?

I'm getting called on my cell phone and my caller id is showing it as old numbers I used to have but I don't have anymore. When I answer, it's a prerecorded message saying I have outstanding tickets or that my shipment of porn with titles x and y and z are being shipped. Does anyone know what service this person is using to do this? I have a friend who knows how to spoof caller id with a pbx, but this seems more than just that, because this has a recording. My friend is not the one making the calls by the way; I already know that.