One of the areas that causes the largest amount of headaches when designing an Enterprise Telephony (ET) network is the dial plan. The causes of these head pains might be due to the complex issues of integrating disparate networks. Many of these disparate networks were not designed for integration.
A good data example of joining disparate networks is when two companies merge. In such a scenario, the companies' data networks (IP addressing, ordering applications, and inventory database) must be joined. It is highly improbable that both companies used the same methodologies when implementing their data networks, so problems can arise.
The same problems can occur in telephony networks. If two companies merge, their phone systems (voice mail, billing, supplementary features, and dial-plan addressing) might be incompatible with each other.
These dial-plan issues also can occur when a company decides to institute a corporate dial plan. Consider Company X, for example. Company X grew drastically in the last three years and now operates 30 sites throughout the world, with its headquarters in Dallas. Company X currently dials through the PSTN to all its 29 remote sites. Company X wants to simplify the dialing plan to all its remote sites to enable better employee communication and ease of use.
Company X currently has a large PBX at its headquarters and smaller PBX systems at its remote sites. Several alternatives are available to this company:
- Purchase leased lines between headquarters and all remote sites.
- Purchase a telephony Virtual Private Network (VPN) from the telephone company and dial an access code from anywhere to access the VPN.
- Take advantage of the existing data infrastructure and put voice on the data network.
Regardless of which option Company X chooses, it must face dial-plan design, network management, and cost issues.
Without getting into great detail, most companies must decide on their dial-plan design based on the following issues:
- Plans for growth
- Cost of leased circuits or VPNs
- Cost of additional equipment for packet voice
- Number overlap (when more than one site has the same phone number)
- Call-flows (the call patterns from each site)
- Busy hour (the time of day when the highest number of calls are offered on a circuit)
Depending on the size of the company, the dial plan can stretch from two digits to seven or eight digits. It is important that you not force yourself down a particular path until you address the previous issues.
Company X plans on sustaining 20–30 percent growth and decides on a seven-digit dial plan based on its growth patterns. This choice also cuts down on the number overlap that might be present.
Company X will have a three-digit site code, and four digits for the actual subscriber line. It made this decision because it does not believe it will have more than 999 branch offices.