For smaller companies, configuring a dial plan to account for variations in dialed numbers is often not a problem. However, companies that are larger and more global in nature encounter a number of issues when designing their dial plan approach.
When an individual travels from the head office in one country to an office in another country, he encounters the problem of having to know which codes need to be dialed to call back to the head office. Here in North America, it is quite easy. We dial an access code to reach an outside dial tone (usually it’s nine), and then we dial the international code of 011, followed by the country code and the number of the individual we wish to reach. But each country has its own codes for international calls and they are certainly not consistent with what we dial in North America. For example, some countries have you dial a “0” for outside dial tone, followed by “00” for an international call, while other countries have a totally different scheme.
Since the number presented to the local gateway must be in the local country’s numbering plan format, a large company needs to add to its dial plan all the possible variations for each countries dialed numbers. This results in some companies having a VERY large dial plan with hundreds or thousands of patterns, route lists, and route groups to deal with all the variations.
Part of the solution is to use the E.164 numbering plan as the main dial plan format. E.164 is the global numbering plan. It defines the unique country codes for all countries and specifies that if a number starts with the “+” sign, then what is expected to follow is a valid country code, followed by a number that defines an individual within that country.
If all the numbers in a Cisco Unified Communications Manager (CUCM) dial plan were in the “+” format, then it wouldn’t matter if the destination phone was in the U.S., Germany, China or anywhere else. The system would uniquely recognize all numbers across the globe.
Now, I know what you are thinking… if you are calling a colleague across the hallway, you surely don’t want to be dialing a whole bunch of numbers…you want to just dial their extension which might be four or five digits long. The good news is that you still can. Usually, when deploying an E.164 dial plan, all directory numbers, route patterns and other internally used numbers are defined in the full “+” format. But the dial plan also uses a number of translation patterns that allow for localized use of extensions.
So an individual can still dial numbers in the format they know works locally. But as you travel across the globe, you should be able to reach any number you normally can by dialing a “+” followed by the country code, followed by the individual’s number. CUCM will then match that number against a directory number or a route pattern which will point the call to the appropriate gateway. Once the outbound gateway is chosen, any transformations of the number will be applied based on the gateway’s available called and calling party transformation patterns. The number actually sent to the gateway will reflect the country specific numbering plan format.
So you can dial +14165551234 in the U.S. or in any other country to reach the desired individual. CUCM will then convert the number to the correct format for the U.S. gateway or to the correct format for the non-U.S. gateway, depending on the outbound gateway choice.
The final benefit to this approach is that as you add more sites to the dial plan, you only have to define the local formats for that site, as opposed to worrying how this site will be reachable from all the other sites you already have.
In summary, using the “+” E.164 format for your dial plan approach allows for easier dialing as staff travels around the globe, results in a smaller number of dial plan elements and makes adding new sites more of a cookie-cutter approach.
ICOMM - Introducing Cisco Voice and UC Administration v8.0
CIPT1 - Implementing Cisco Unified Communications IP Telephony Part 1 v9.x
ACUCW1 - Administering Cisco Unified Communications Workspace Part 1: Basic v9.x