Monday, July 19, 2010

What is DID number and why we need it?

What is a DID number
A client asks about DID number. 
We are selling DID numbers in Hong Kong (5804-) and China (400-) to overseas call centers.  A number of inquiries have been received.
In the first place, what is a DID number?  It stands for Direct Inward Dialling number (    You purchase a DID number from a telecom operator in another country and forward it to your local contact number so it looks like that you have a point of presence in the foreign country.
It becomes useful when you are operating a call center in e.g., US and would like your Hong Kong customers to call a Hong Kong number to reach your agents.  There are many reasons for not having an overseas call center, such as high labour cost, high setup cost, difficulty with remote management or lack of agent expertise.   With DID, you could serve customers around the globe with centralized resource.
The DID number is usually forwarded to your local contact number via Internet and so you usually do not need to pay the call charges for air-time.  The DID provider usually just charges you a basic monthly fee per number per channel plus an once-off setup charge.  A number with single channel means it could handle one simultaneous call only.  The second caller will hear busy tone.
Value added services
Some providers offer voice recording as an customizable option.  Conditional forwarding and automatic call distribution (ACD) will add more flexibility and control to your call center.  These are fundamental features in many of the call center systems today.  However, if you are just about to start a small call center with limited system investment, these on-demand value-added features are worth your consideration.
Technical consideration
While we mention that DID calls are forwarded via Internet, we have to know more about the protocol and bandwidth required.   The forwarding over Internet is accomplised by a standard VOIP protocol, Session Initiation Protocol (SIP).  This protocol controls the signaling and voice communication sessions over IP.   Your IT support team should be able to manage a SIP based PBX system, like Asterisk or Trixbox to connect the incoming SIP trunk from your DID provider.
In addition, to conserve internet bandwidth, audio data is usually compressed to 8kps bitrate using the G729 codec.  Practically, a two-way conversation consumes about 70-80kbps bandwidth.  This figure takes into account the payload and overhead in transmission.    You might have to check your ISP to allocate sufficient and reliable bandwidth for your agents. 
Note that DTMF tones and fax transmissions could not be reliably transported with G729.


  1. local china DIDs with more than 2 channels. Does anybody provide this service?