trixbox Pro supports three different types of voicemail greetings configurable via the voicemail system.
Your unavailable greeting will be used when you phone rings for a specified period of time without being answered. As the name implies, this greeting is meant to play when you are away from the phone.
Your busy greeting will be used whenever you are on the phone and a second call comes through to your extension. If you choose not to answer the second call, the caller will hear your busy greeting after the default ringing period (configured by your administrator)
Your vacation greeting supercedes both 'unvailable' and 'busy' greetings and plays for all incoming calls after the default ringing period (configured by your administrator). The vacation greeting is intended for holidays and sick days. Once recorded, it is automatically active for your voicemail box. You must disable the greeting via the voicemail system in order to restore your unavailable and busy greetings.
No - each greeting type is recorded into an independent sound file. They are not dependent on each other.