VoIP providers have to keep the call records for their customers says EU. The move is contentious, but possible necessary. However, they are also making them keep up with the missed calls:

Tracking dropped calls has been particularly controversial, since service providers don't currently register lost calls for billing purposes. In fact, the technology to track lost calls requires new, expensive technologies.

This seems a bit excessive. Now I'm not a legislator, nor a terrorist, so I have no first-hand information, but do hangups usually require tracing for criminal activities?