Indicates the capability of an extension (ephone-dn)
to process message-waiting indication (MWI)
notification from an external voice-messaging systems.
mwiOn - Extension can process MWI ON notifications
using either the main or secondary phone number.
mwiOff - Extension can process MWI OFF notifications
using either the main or secondary phone number.
mwiOnOff - Extension can process both MWI ON and OFF
notifications using either the main or
secondary phone number.
mwiDisabled - MWI notifications are disabled on
an extension.