Multiple Subscriber Numbers (MSNs) are additional numbers that can be associated with the main number of your ISDN circuit. They allow you to allocate different numbers to devices such as telephones, faxes and computers. You will need to program the equipment you connect to the ISDN line so that only the correct device responds to its allocated MSN.
MSNs are available for ISDN lines operating in point-to-multi-point configuration (also known as 'standard access' or 'S/T reference-point configured lines'). Point-to-multi-point working is normal for ISDN terminal adapters, PC cards and telephones. If you are using a PBX, you may need to use DDIs, see below.
Direct Dial In (DDI) Numbers are additional numbers that can be associated with the main number of your ISDN circuit. They are normally used with a switch/PBX. DDIs can be allocated to extensions, providing separate numbers for individual staff. You will need a switch/PBX that can be programmed to route incoming calls using DDI numbers to the correct extensions.
DDIs are applied to ISDN lines operating in point-to-point configuration (also known as 'system access' or 'T reference point configured lines'). You should check that your equipment supports this.
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