You need to make a mailbox with the same number as your ring group like 191 you shouldn't be working with mailbox 998 or 999 unless your moving the messages to those mailboxes the settings you need to change are for mailbox 191.

And change operator dial 0 to ring 191 instead of 101

You can change the 816 to use ringback on transfer or play MOH it is in additional function programming.