Can I use DID numbers with SIP trunking?

By Brian Boguhn
Updated: February 03, 2011

You’ve decided to go with SIP trunking to lower costs. You’re getting ready to implement the solution. One of the questions that arise is whether or not DID numbers can be used with SIP trunking.

Direct Inward Dialing

DID is an acronym for Direct Inward Dialing. It is a feature provided by communications companies for use with the PBX (private branch exchange) system, or traditional phone system, located on a customer’s premises. One or more trunk lines come from the provider into the customer for connection to their PBX. A range of telephone numbers is then allocated to this line or group of lines. Any time a phone number in the group is dialed, the call is forwarded to the number via the trunk. DID allows the call to be routed via the PBX to the extension within the company without any sort of human intervention. Put simply, DID permits inward call routing to any extension within the company even though a limited number of subscriber lines are maintained.

It is now apparent why this feature would be attractive to companies using SIP trunking. The good news is that SIP trunking does indeed support DID numbers. Not only can you have multiple DID numbers routed across the trunk and then to the correct destination, you can include local numbers around the nation as part of your DID collection. Customers will be able to dial a local number in their area and be routed to the proper recipient, even if that recipient is physically located somewhere else in the world.

What's Needed

The prerequisites for using this service are pretty straightforward, and include some of the items needed when you implement SIP trunking. You’ll need a high speed connection to the Internet to start. Keep in mind that this connection will carry both data and voice, so it would be wise to talk to any potential provider and make sure that you have enough bandwidth available. You’ll also need an IP-PBX with an SIP ready trunk. The IP-PBX will handle all calls on the IP network and communicate with the SIP trunk. It will switch all calls as needed and allow users to share a number of external phone lines. If your provider can’t make your existing PBX work with SIP trunking by use of an Integrated Access Device (IAD), you may need to purchase an IP-PBX. Finally, you’ll need DID numbers.

The VoIP gateway in the system will handle all translation as well as directing calls. Any call placed through the public switched telephone network (PSTN) will be converted by the gateway into digital data packets that are compatible with the VoIP system. Built-in routing tables will help to route calls to users. The routing tables will need to be programmed, and it is here that the DID numbers come into play:

  • You can configure your DID numbers to go to individual users, or
  • You can create hunt groups to ring multiple users.

This is very similar to the programming done in a traditional PBX.

Conclusion

DID numbers have traditionally made life easier for companies. Having this feature available for SIP trunking continues that tradition. You save money by using the trunking, and still enjoy the ease of use you’ve grown accustomed to in the past.

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