30-Second Summary:

  • Today, most enterprises are composed of main sites, branch offices, and even a mobile workforce.
  • This has resulted in a multi-site environment where employees are distributed throughout a city, a country, or even around the world.
  • VoIP offers flexible solutions that allow for a multitude of different combinations of connectivity for voice services.
  • This essentially means that telephony services can be deployed anywhere where there is an IP connection.
  • Even with these wonderful advantages, there are certain challenges that must be considered before designing a multi-site voice deployment.
  • Read on to understand and learn exactly what it means to deploy VoIP in a multisite environment.

Introduction

Several decades ago, it was quite common for successful businesses to operate out of a single physical brick-and-mortar location where all of its employees would gather daily to perform their various tasks.  Since then, the business landscape has changed dramatically.  Today, most enterprises are composed of main sites, branch offices, and even a mobile workforce.  This has resulted in a multi-site environment where employees are distributed throughout a city, a country, or even around the world.

Deploying voice services in such a multi-site environment can be challenging, depending upon the services being leveraged.  In this article, we hope to shed some light on these challenges and to also describe the most common best practices that are used to deal with them.

What is a multi-site environment?

Today’s workforce is no longer tied to a desk.  VoIP offers flexible solutions that allow for a multitude of different combinations of connectivity for voice services.  This essentially means that telephony services can be deployed anywhere where there is an IP connection.  In today’s connected world, IP connectivity is virtually ubiquitous.

Multi-site network environments take advantage of this connectivity by allowing telephony and voice services of all types to be transmitted directly from one location to another without going through a telephony provider and being charged.  This essentially creates an internal voice network that can have many advantages including:

  • Three-, four-, or five-digit dialing plan for all internal calling, depending upon the size of the organization.
  • Free communication between sites, no matter where those sites may be located worldwide.
  • Tail end hop-off feature, allowing call routing that will minimize telecom costs and enable you to appear to be a business with a local number, even if you’re half a world away.

Even with these wonderful advantages, there are certain challenges that must be considered before designing a multi-site voice deployment.  These include:

  • Overlapping dialing plans, in the event, that some shortcode calling numbers are the same at multiple sites.
  • The establishment of reliable WAN connections that interconnect the remote sites to ensure high availability of the service.

Multi-site network environment examples

Below is a simple diagram of a multi-site network of an international corporation with headquarters located in the US and in Europe, plus two branch offices in the US and one in Europe.  You can see that each physical location has a connection to the public switched telephone network (PSTN) through which outgoing calls can be made.  However, it is possible to route VoIP services between branches and headquarters via the WAN and VPN connections bypassing long-distance and international toll charges.

In addition, a short code dialing plan can be devised so that internal calling does not require seven- or ten-digit dialing. 

VoIP implementation methodologies

For smaller businesses, it may be simpler to use a service such as Ooma that delivers a PSTN number to either a mobile phone app, a desktop app, or to a pre-provisioned IP desk phone.  Communication between such devices would take place using the regular PSTN number and may or may not incur charges, depending upon the provider.  Such an arrangement is especially useful if you have a mobile workforce and most of your employees are within the same area code.  That said, if your service exists in different cities, or different countries, then there will definitely be service charges for communication between these devices.

For larger enterprises, the most cost-effective solution would be to deploy a multi-site network like the one shown above with SIP trunks being used for PSTN connections.  Many companies, including RingCentral, deliver SIP trunks as a service option.  These trunks allow local IP PBXs to connect to the PSTN in much the same way as more conventional ISDN PRI circuits connected traditional PBXs, but with a great many advantages.

This results in multiple internal IP telephony devices, each corresponding to an external PSTN number, but also to an internal short code number, thus enabling short code dialing.  VoIP routing can then be configured to send the appropriate shortcode dialing trains to the correct branch/remote site to be terminated on the right device.

Multi-site telephony design concepts

Below are some of the concepts and challenges that you will face when designing and implementing a multi-site voice services solution.

Overlapping dial-plan

When implementing shortcodes, you typically use the last few digits of the PSTN number.  For example, if your corporate telephone number is 416-555-7258, a three-digit internal short code would most commonly be 258, which is composed of the last three digits of the number.  This is fine for a single site, but when you have multiple sites, which may also be in different area codes, it is likely that some numbers provided for you by the telco will overlap with other sites.  For example, a remote site may have a device assigned the number 514-555-7258, which would result in the use of the same shortcode for that phone.  The result is that three-digit shortcodes may not be unique.

To resolve this issue, you can do one of several things.  Negotiate with your telco to try to get numbers that don’t overlap.  This is not always easy. It depends upon how close of a relationship you have with the telco and how accommodating they are. 

Another option is to not deploy direct inward dial (DID), where each internal extension corresponds with an external number.  You can eliminate this and simply ask incoming callers to dial the extension number of the person they wish to reach.  This will allow you to have full control over the internal extensions, changing them as you see fit.

If you can’t get rid of the DID implementation, then the other solution is to use site codes.  The diagram below depicts the situation we mentioned before.  Branch 1 is given one thousand numbers in the range of 416-555-7XXX and similarly, Branch 2 is given one thousand numbers in the range of 514-555-7XXX.  Using three- or four-digit dialing will not make these numbers unique.  So a site code can be used.  To reach Branch 2, use a site code of 2.  This acts as a prefix, which means that anyone at Branch 1 must dial 8258 to reach the IP phone at Branch 2.  Similarly, Branch 1 can use a site code prefix of 1, so that the phone at Branch 2 would have to dial 1258 to reach the phone at Branch 1, thus making the numbers unique.

Tail End Hop Off feature

This is a feature that can be very useful especially when you have multiple locations in different areas of the country, and even more so when you have multiple international locations.  Tail end hop-off, or TEHO, is a network design principle that allows you to use your VoIP network to route calls to PSTN destinations where one of your remote locations exists, at local calling rates, where you would otherwise be obliged to pay long-distance or international telephone tolls. 

Imagine a situation similar to the one below:

You are using your corporate telephone at the USA headquarters, and you want to call a business partner that is physically located in Europe, in the same city as your European headquarters.  You could call internationally and incur international calling costs.  However, if your network has been configured with TEHO, your call could be routed through your own network and can be directed to exit the PSTN at the European headquarters location like so:

The destination of your call isn’t on your corporate network, but it can be reached via the closest PSTN connection on your network.  This would result in local telephone costs on the PSTN trunk at the European headquarters rather than international rates if you called from your local PSTN number.

Configuring TEHO requires a coordinated and coherent telephony network configuration throughout your local and remote sites and should be employed taking into account the local calling rates at each location to ensure the cheapest possible rates.

There are a few of things to remember when using TEHO.

  • In the USA, TEHO is legal, but in some countries, it may not be, so make sure that you are abiding by the local laws in each region.
  • The caller ID that appears on a call that uses TEHO will not be that of the caller, but of the local business from which the call terminates on the PSTN.
  • If your system is not configured correctly, you may accidentally route 911 calls via TEHO, which could result in the dispatching of emergency services to an incorrect location.

Conclusion

Multi-site voice deployments can quickly become complicated.  As new sites are added, as companies expand, and as various organizations merge, their telephony systems can become somewhat unwieldy to administrate and manage.  The concepts and principles described here should help you in getting the most out of a multi-site voice network environment while minimizing the problems and challenges that may arise from it.