They are two separate functions. The directory you are seeing is just a mirror of what is stored in your phone. If you want to voice dial by name you need to add the same names and numbers in to the ULFs memory by pressing the talking head button and saying "store name", the system will then ask you for the name and then the number. You will need to do this for each entry.
Once done you can give it the command to "dial name" and it will ask you to speak the name then you tell it to dial.