Presenting information and knowledge about ddi!
So simple yet uncertain. Those who read will find out.
Direct Dialling In.
This allows an extension line on a PBX to be called directly from the public network without operator intervention (i.e. direct lines).
Another definition of ddi:
Direct dialling in (DDI)
Allows external callers into a PABX to dial directly to the required extension without going through a switchboard operator.