Elizabeth (Beth) English
Elizabeth (Beth) English is the founder and lead consultant of EE and Associates, LLC. She brings more than 30 years...
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Elizabeth (Beth) English | May 04, 2016 |


Porting from DID to SIP: Eating the Elephant One Bite at a Time

Porting from DID to SIP: Eating the Elephant One Bite at a Time Everything your business will want to consider when deciding how and when to port DID numbers to SIP trunks.

Everything your business will want to consider when deciding how and when to port DID numbers to SIP trunks.


With the ubiquity of VoIP, unified communications, and SIP trunking, most organizations either have ported or are in the process of porting numbers from DID to SIP. For those not familiar with the term "porting," in this article, it refers to the process of moving DID phone numbers from the local central office they have been tied to for decades, allowing them to move about freely and terminate in a centralized SIP gateway. DIDs, which were previously limited by geography, can be delivered over Ethernet circuits, allowing organizations more choice in selecting a service provider.

Allowing numbers to be delivered outside the confines of LATAs and LEC territories is revolutionary. As with any revolution, there can be substantial challenges with moving from the current state to the end state.

The LECs controlling the numbers are not keen on giving up revenue gained from producing DIDs, and practically by default, they can make the process as cumbersome as possible. Most will not guarantee a time of day when the porting will occur, putting business at risk during the porting process. Some will refuse to port the Billing Telephone Number (BTN) as long as there are non-DID trunks on the account, forcing customers to complete a change order (which could take weeks) prior to submitting a port order. Orders can be rejected for minor differences in the address such as spelling, punctuation, and capitalization, or if there are other orders in process on the same account.

Whether your project is a simple conversion from DID trunks to SIP on an existing system, or a complex, multi-location project involving both systems and trunk conversions, given the complexity of porting numbers, careful planning and consideration should be given to how and when porting is done.

If working with a single location, existing system, or if porting from a single location old system to a single location new system, it is logical to port all the DIDs in a "flash cut." Placing one order with the LEC and coordinating the cutover for a day when resources are available is the least painful alternative. Keeping in mind that many carriers will not guarantee a time of day numbers will port, all programming should be done and tested in advance of the cutover day. Done right, the conversion will be transparent, with minimal downtime as calls stop flowing in over T1s and begin flowing in over SIP trunks.

If you have a multi-location configuration with buildings separated across a campus or distributed around a state, scattered across country, or flung around the world, however, when is the best time to port DIDs to SIP trunks?

When to Port

As an example, consider a scenario where you have a campus environment with buildings all served out of the same CO, on a common dial plan, distributed throughout a city, with different NXXs with end users moving from a traditional PBX to a unified communications platform. In this case, what is the best plan for moving DIDs to centralized SIP trunks?

As each department cuts over to the new system, the associated DIDs will need to terminate in the new system. A sizable project is unlikely to convert all buildings and departments at once, but rather will stage them. If there were 5,000 DIDs, and 500 moving each week for a period of eight weeks, followed by a five week period of moving smaller units of 200, then the project would span 17 weeks.

In this example, when should DIDs be ported?

Technically speaking, numbers can be ported in bulk, all at once, or in small groups as each group is moved onto the new system. At first glance, the logical option may appear to be to port the numbers as you need them. Realistically, however, while porting intact blocks of numbers with LECs can be cumbersome, trying to port individual numbers can be downright arduous.

In another scenario, if you have a block of 1,000 DIDs, and you only need to port 625 of them which are not contiguous, is it better to port only those numbers you need at the time you need them to avoid paying for unnecessary numbers? Or should you port them all and then disconnect what you don't need?

In a hosted unified communications environment, if the receiving provider charges $8 per line to port numbers in, plus $1 per month per DID, porting only the 625 individual numbers you need may look like a good choice.

In some cases, however, the "losing carrier" may charge a service or programming fee per DID to split the DID block. If the losing carrier charges $25 per DID for the remaining DIDS, the cost of splitting the block with the current carrier may quickly exceed the costs charged by the receiving carrier. These fees need to be factored into the calculation. In this case, the more prudent move would be to port all of the numbers, then disconnect those not needed with the new carrier.

Consider Your Options

Once you decide to port all of the numbers at once, the next question is at what point during the project should they be ported? At the beginning, middle, or end?

One option is to port all of the numbers at the beginning of the project so that they terminate with the SIP provider in the new system. Tie trunks can then be established between the new and the old system, with the new system sending all DIDs to the old system over the tie trunks. As users are moved onto the new system, forwarding for the individual DIDs associated with those users is removed.

Moving all of the numbers at the start of the project offers some advantages. If programming changes are needed, they can be accommodated quickly, rather than having to wait weeks for a LEC order. Given the historically poor track records for LECs cooperatively porting numbers, particularly when numbers are being ported away from them, having control of the numbers early in the process ensures control of the numbers lies with carriers and service providers that are more invested in the project's success.

Moving all the numbers early in the project, however, also increases the risk that the new system is not fully stabilized and the technical staff has not had sufficient time to become familiar with the new technology before moving over to SIP.

A second option is to leave the DIDs with the existing carrier until the end of the project. With this option, all DIDs continue to terminate in the old system. As users are moved onto the new system, individual DID numbers are forwarded to the new system over tie trunks. Once all users are on the new system, the block of DIDs is ported in bulk to SIP trunks.

Moving numbers at the end of the project allows technical staff to become fully qualified on the new system before attempting to make the move. Technical staff can focus on the user experience, getting everything working on the new system, before introducing the complexities of SIP trunking. The new system can be test driven, working out any issues. Once the system is stabilized, the numbers can be ported all at once, allowing new issues that arise as a result of porting to be isolated to SIP trunks, SBCs or network issues.

This option leaves control of the numbers with the existing carrier throughout the duration of the project. The associated risk of this approach depends on the cooperation of the current carrier. As the time approaches to make the final transition from old carrier to new, the current carrier will be less incented to cooperate for the success of the project.

A third option is to port the numbers at some midpoint in the project. Similar to porting at the beginning or end of the project, tie trunks are required to direct numbers between the systems. Porting the numbers at a midpoint offers the same advantages of porting at the beginning and at the end. The new system can be stabilized, and the team can become familiar with it. At a chosen point part way through the project, the numbers can be ported from DID to SIP. The team, now familiar with the new system, has control of the numbers before relationships with the losing carrier and old systems maintenance folks deteriorate.

If the technology does not support tie trunks between the old and new systems, the only option may be to port the numbers as needed. In that case, having a good inventory and strong project lead dedicated to keeping track of number inventory, port order, and vendor management is critical to the success of the project.

While there is not a one size fits all answer to when and how to port DID numbers to SIP trunks, there are criteria which can be used to determine which alternative is best for your organization.

"SCTC Perspectives" is written by members of the Society of Communications Technology Consultants, an international organization of independent information and communications technology professionals serving clients in all business sectors and government worldwide.


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