This may not be the best solution but it is one I have used before, if the Aastra phones aren’t able to provide the unavailable command then this may be an option.
At the bottom of the extensions page in the voicemail section the three boxes say something like:
“unavailable message if enabled”
“busy message if enabled”
“unavailable message if enabled”
You can change the middle destination to voicemail>same extension number (unavailable message)
Now this will always play the unavailable message, even if you reject an incoming call, which may not be what you want.