Centralizing Call Flow

Knowing Your Telephone Numbers

For many organizations, the ability to centralize call flow through a primary location proves to provide significant cost reductions for several reasons.

  1. centralized-call-flow Rather than managing distributed trunks groups across an enterprise balancing excess costs for maximum call volumes vs. callers receiving busy signals, a centralized trunk group can minimize this concern.
  2. Organizations can eliminate expensive hardware at the remote sites replacing it with less expensive gateway equipment.
  3. Remote site employees can be re-focused on other critical tasks when not having to answer all incoming calls.

Centralizing call flow can generate substantial monthly cost savings up to 60%.  And, when implementing new telephone systems, capital costs can be decreased up to half.  However, unlike distributed voice trunks, aggregating these lines can create noteworthy challenges if not accounted for in the planning stages of the deployment.

One of the most important elements is understanding the different types of telephone numbers that SIP can offer.  There are generally three types of telephone numbers available with SIP trunking.

  1. Standard Direct Inward Dial (DID) numbers
  2. Virtual Telephone Numbers
  3. Enhanced Virtual Telephone Numbers

In general, a physical location is surrounded by boundaries based on a local telephone company central office.  Traditionally, only telephone numbers within these boundaries can terminate at the physical location.  These are standard telephone numbers or DID’s.  The illustration below shows these boundaries for a four location organization.


In a centralized call flow environment where telephone numbers for all the sites are terminated at a single location, SIP trunking has the capability of using virtual or enhanced virtual telephone numbers to account for this.  With SIP, almost any telephone number can be rerouted to this centralized end point.  However, the selection of the method that these remote site telephone numbers are forwarded is critical.

Both virtual and enhanced virtual telephone numbers can have the same general impact.  A caller would dial the telephone number and, rather than it terminating at a remote site, the call is redirected to the central site.  The significant difference is in relation to the use of the specific telephone number for outbound calling.

Enhanced Telephone Number Characteristics

Depending from which site an outbound call is originated, the centralized telephone system will outpulse a telephone number from the respective range of telephone numbers.  This is the telephone number that will be shown on the called party’s caller ID.  Bulleted below are some specific characteristics belonging to Enhanced Virtual Telephone Numbers (EVTN)

  • E911 – Because it is critical that emergency services arrive at the correct location, an EVTN ensures that the correct Public Safety Answering Point (PSAP) is engaged. The EVTN is registered with the carrier along with the associated physical address so this occurs.
  • Local Calling Area – The EVTN also ensures a call made from a remote location to another party local to the site is rated as a local call vs. at long distance rates. Although the reduction in long distance rates over time has decreased significantly, there is still a deviation and ultimate monthly cost savings by applying local calling rates.
  • Called Party Name Display – There are some organizations that, although belong to the same organization, have different names to be dicentrsplayed on the name display of whom they are calling. This can apply to multiple locations with unique names or a single site that hosts many organizations.

Telephone Number Costs

The question often arises, why not just set up all the telephone numbers as EVTNs?  The only reason would be monthly and one-time potential costs.  Depending on the carrier and the associated service package, different monthly and one-time costs can apply on a per telephone number basis.  So, the determination of how each telephone number is implemented is a balance of cost, flexibility and future expected needs.  And, it is important to evaluate how each of the carriers rate these telephone number types because they will vary.


Many organizations with multiple locations are finding the centralization of call flow into a primary location can reduce monthly costs significantly, up to 60% per month.  However, it is vital that the telephone numbers are set up accordingly with the carrier to ensure the end result is the desired outcome.  And, the telephone system must be able to support what you are trying to accomplish to ensure the correct telephone number is outpulsed dependent on the originating location within your enterprise.