Someone from this number calls about four or five times a day; every hour or so and when I pick up... they hang up. Any Ideas?

This has been going on for about a week. It started suddenly and stopped on Sat and Sun then started again yesterday (Monday). The calls start coming in about 10 or 11 am and continue until 3 pm or so.

The phone company (Quest) says to keep track of the calls and they will notify the caller that this is harrassment. But they can not tell me who it is that is calling. Something about privacy.