There is no voice dialing on the n900, and the Jabra Cruiser says this feature is phone dependent, so it wont work. Also if your using two devices that support voice dialing on the Jabra Cruiser only the primary device will work, just for your information.

I don't know how the Jabra downloads the phonebook, but it is probably from the SIM, since the Jabra documentation says it is limited to 1000 entries, which is a SIM card limitation. Unfortunately the n900 doesn't store it's phonebook on the SIM automatically, but there is an option in the Contacts application to copy contacts over to the SIM.