DDI (Direct Dial In) is a set of additional numbers available if you have a PBX (private branch exchange) so that outside callers can direct dial into PBX extension telephones from the public switched telephone network (PSTN).
DDI (Direct Dial In) is a set of additional numbers available if you have a PBX (private branch exchange) so that outside callers can direct dial into PBX extension telephones from the public switched telephone network (PSTN).