SMDI can be used over RS-232 to connect a PBX to a voice mail system. SMDI is an out-of-band signaling which necessitates voice and signaling information being carried on separate paths.

The integration is possible via RS-232 connection because it transports the Automatic Number Identification (ANI) and the Redirecting Dialed Number Identification Service (RDNIS) which are used to indicate the calling party number and the original intended receipent respectively.

SMDI is used to transmit information about call types, reason of forwards, about MWI On/Off states and about error states. SMDI messages are comprised of ASCII characters and symbols.