Greetings-Only voice mailbox support added.

• 0 is allowed for maximum number of message for service and phonelines.
• Setting 0 as maxmsg for a voicemail box means it is greetings only.
• Message playing rule:
  
  • if a vm box is greetings only the caller will listen to recorded 'unavailable message' for that mailbox,
  • otherwise it will play the recorded 'name' for that box + "is unavailable" message
  • if none of the message is recorded for the box it will declare the box number + "is unavailable" message