Dial Plans 101

April 24, 2017 Collaboration

Dial plans determine how calls move into, within, and out of a phone system.  They determine how many and what digits must be dialed.  This is a very important component of a voice deployment that can become very complex so my goal is to break down two of the most complex components when it comes to considering dial plans to readers better understand them.

Digit Length: The biggest consideration of a dial plan is the length of the numbers assigned to a phone.  In North America, we use the North American Numbering plan.  In this plan numbers consist of 10 digits.  Other countries can have more, less, or variable length digits.  Let’s take a number 8045551212 as an example.  We can choose to use the last 4 digits of the number for our phone and add or strip the 804555 at the gateway to the phone company.  This works well in small systems.  Everyone dials 4 digits to reach internal numbers and an access code followed by the full number to reach outside numbers.  The problem comes when there is an overlap of digits.  Let’s say we have a phone at our headquarters location that ends in 1212 and a phone in our plant location that ends in 1212.  In this case, we can’t dial 1212 and reach the right phone.  We must change something.  One way to do this is to have the phones at headquarters and the phones at the plant operate separately.  To do this, we partition the system so that calls from headquarters don’t reach the plant and vice versa.  Then to reach the plant, we dial a site code, followed by the number.  This works, but it has some disadvantages such as the fact that it doesn’t scale well and people must learn the site codes and that list can become too long.  A better option is to use more of the actual phone number as the extension.  For example, if we use 8045551212 as the extension, we can’t have any overlap and don’t need to worry about site codes.  This helps simplify the dial plan.  We can still have 4-digit short cut extensions where numbers don’t overlap by creating partitions.  This is an excellent way to work with large multisite situations in the same region.

International Deployment: The next problem we run into is to consider what should be done in large international deployments?  In this situation, we must deal with the number and the country code.  For example, 1 followed by 8045551212 for the United States and 49 followed by a variable number of digits for Germany.  To avoid overlap, we must use the whole number including the county code.  This is the E.164 numbering plan.  +E.164 refers to numbers prefixed with a +.  This indicates a complete E.164 number.  The big advantage of +E.164 numbers is that they are globally unique.  This allows a system to deal with all internal numbers without the need to partition them out.  It also helps with things like redialing from a missed calls list.  If you have the complete E.164 number in the list, it is very easy to just dial it back to call out.

Obviously one may now see that careful consideration when it comes to generating a dial plan is important.  Overlapping of numbers adds complexity to the dial plan and should be avoided.  For small sites, a 4-digit dial plan may work fine.  For larger multi-site deployments, it’s likely that longer numbers will need to be used to avoid overlap.  Using the full North American Numbering Plan number is an example.  For multi-site international deployments, the full +E.164 number is likely going to be required.  The important thing to think through is where the numbers will overlap and how that can be avoided going forward.  Furthermore, it’s always a good idea to plan for growth as well.  Once a numbering plan is in place and many phones have been configured along with patterns to reach the outside world, it is time consuming to make changes.  It’s always best to plan carefully up front from all angles.


Curtis brings over 25 years of collaboration experience to ABS. As the Collaboration Team Manager, Curtis works to ensure that ABS is consistently providing the latest collaboration technology and support to ABS clients.