Possibly because you dont have a VM box or VM profile configured for the attendant phone. What happens when you press the VM key on the 79xx/desktop phone? Sem thing?
If theres nothing configured, try adding the VM profile (ie transfer to Unity) to the Directory Number configuration page for the attendant DN
no need to hunt or forward no answer to voicemail if you dont want calls to route VM (since its the attendant), you just want to define a VM profile so callmanager knows where to route the call when the VM key is pressed