Learn about DID, DDI and MSN numbers and how they work

Posted by claire on April 4th, 2019

What is DID?

DID refers to Direct Inward Dialling. It is a feature which is provided by phone companies for the using with customers Private Branch Exchange (PBX) system. With DID a business can provide its employees with separate phone numbers for every workstation or person with in the office.

How is DID used with VoIP?

For the longest time in order to obtain a phone number the only mean was an operator who physically allocated telephone lines to the PBX system. On the contrary, now all this can be accomplished with the VoIP and internal networks. Using DID is cheap, fast and comparatively easy.

If a business wants to use DID along with VoIP technology, then they have to do this:

  • Buy a PBX software package which meets all your need best

  • Search for a company that can provide DID phone numbers

  • Download the software on a server

  • Test the system and go live

How is it helpful?

A DID service provider can help you in buying virtual numbers. With a virtual phone number, you can make and receive calls to you VoIP service app or you can redirect calls back to your landline or mobile numbers. This way the caller won’t know your real phone number or the redirection that happens internally.

For Example: A business “ABC” established in the UK has customers in the UK, but its customer support services are in India. The company cannot expect its customers to make international calls from the UK to India for customer support. Therefore, the business will buy a virtual phone number from a DID service provider which offers UK local phone numbers. A DID provider will then map all of the incoming calls to the UK local number to the India Call Centre through VoIP services or through GSM network.

What are MSNs?

MSNs stands for Multiple Subscriber Numbers. These numbers are related to the Integrated Digital Services Network (ISDN). These numbers enable users to assign separate phone numbers to different devices with in the network. Similar to landline, a whole MSN includes an area code and country after that the MSN number.

What are MSNs used for?

MSNs are utilized to make sure that each device within the network has different phone numbers. In additions these numbers allow users to make multiple calls at a time through a single ISDN connection. Whenever a call passes the ISDN network, it is transported to the particular MSN through the S0 bus or S interface.

What is ISDN?

ISDN stands for Integrated Services Digital Network. This technology has brought the digital functionality to traditional telephone network, also known as Public Switched Telephone network (PSTN). ISDN particularly allows simultaneous transmission of different kinds of data like video and audio over the PSTN.

How are placing calls different on MSN compared to VoIP?

VoIP services are comparatively more flexible compared to MSNs. This is due to MSNs static numbers which are assigned to fixed points in the network, while the UK VoIP number and VoIP services enable users to login from any place through any device. Meaning with MSNs the employees who visit different offices and aren’t able to use their phone numbers can just communicate through local phone numbers at the office. With the VoIP phone solutions such employees just have to login to the remote office network and can use their own phone number in order to contact their customers, clients or colleagues.

What is the difference between MSN and DDI numbers?

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.


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Joined: April 4th, 2019
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