Voicemail server needs to use /etc/event.d/ttyS[n] to start vgetty. Upstart will not start it from inittab.
Voicemail server will show no serial ports are configured without the line in inittab, but this line will be ignored by the system.
My /etc/event.d/ttyS0:
# ttyS0 - vgetty
#
# This service maintains a vgetty on ttyS0 from the point the system is
# started until it is shut down again.
start on runlevel 2
start on runlevel 3
start on runlevel 4
start on runlevel 5
stop on shutdown
respawn limit 10 300
respawn
exec /sbin/vgetty ttyS0