Asterisk SMDI (Simple Message Desk Interface) integration
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SMDI integration is configured in smdi.conf, zaptel.conf, and voicemail.conf.
Various characteristics of the SMDI interfaces to be used (serial ports) are
defined in smdi.conf.  SMDI integration for callerid and MWI are defined in
zaptel.conf and voicemail.conf respectively. SMDI only works with Zaptel
interfaces configured for FXS signalling.

When SMDI is enabled and a call comes into Asterisk, the forwarding station
number is used as the destination for the call and any callerid information
present is used.  This way you can configure your extensions.conf as follows to
behave as a message desk.

[default]

exten => _XXXXXXX,1,VoiceMail(${EXTEN}|${SMDI_VM_TYPE})
exten => _XXXXXXX,n,Hangup

exten => s,1,VoiceMailMain(${CALLERID(num)})
exten => s,n,Hangup

The ${SMDI_VM_TYPE} variable will be set to u, b, or nothing depending on the
contents of the type of SMDI message received.