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