Simplified Message Desk Interface (SMDI) is a protocol that defines the interface between a voice mail system and a phone system such as a PBX or public telephone switch.
The SMDI protocol also specifies a method for turning the message-waiting indicator (MWI) on and off on individual telephones.
Some of the more recent voicemail systems transmit the SMDI protocol over a network TCP session rather than a serial interface.
[2] According to Telcordia GR-283-CORE, the Simplified Message Desk Interface (SMDI) is an analog interface between the local stored program controlled switching system (SPCS) and a message storage and retrieval (MSR) system.
An identification scheme is assigned in the SPCS to identify each MLHG uniquely and each line in the MLHG, for which call information (e.g., calling number) is provided over the SMDI data link.