Multiple Subscriber Numbers (MSNs)
Direct Dial In (DDI) Numbers
MSNs are the additional phone numbers which can be related with the main number of the ISDN circuit.
DDIs are the additional phone numbers which can be related with the main phone number of the ISDN circuit. These numbers are usually used with a PBX/switch. Users can buy DDI number to work with their main phone number.
These numbers allow users to assign different phone numbers to different devices such as faxes, computers and telephones.
A DDI number can be assigned to extensions which provide separate phone numbers for individual staff.
Users will have to program the equipment which they connect to the ISDN line, this way only the right device replies to its assigned MSN.
Users need a PBX/switch which can be programmed to route incoming calls through DDI numbers to the right extensions.
MSNs are available for the ISDN lines which operate in point to multi point configurations also called ‘standard accesses. Point to multi point working is usual for the ISDN terminal adapters, telephones and PC cards.
DDIs are applied to the ISDN lines which operate in the point to point configurations also called ‘system access’ or T reference point configured lines’). Users will have to check whether their equipment supports DDIs.