3 - Call history feature Edit: Same as 13
4 - Add register status information
8 - allow forward to mobile or fixed line number via one of our providers.
12 - User definable Voicemail answer delay and on/off.
21 - Dial plan fail over. i.e if dialling an entry fails, look for the next matching priority (if one exists) and try that one.
The Doc
OOPS... I've just seen that the votes are over... and I've chosen the 5 with more votes! Lucky me!