86 CHAPTER 2: D IAL P LANAssess your growth plans for each site to verify that as you addtelephones you do not exceed your defined extension ranges.Figure 10 depicts a configuration that uses unique extension rangesFigure 10 Multi-site Network using Virtual Tie LinesIn the sample network shown in Figure 10, each site is set up to use aunique range of telephone extensions. The dial plan on each of thesystems is configured so that whenever a call is made to an extension notlocated at the local site, the NBX system sets up a VTL connection to theappropriate site.To make a call to a user in Dallas, a user in Chicago dials a Dallasextension (3000 through 3999). The dial plan on the Chicago NBX systemis configured to set up the necessary VTL connection to the Dallas NBXsystem, and then to the extension at that site.See “Dial Plan Configuration” on page 90 for further information onhow to set up VTLs in the dial plan.VTL ConnectionsUsing Site CodesThe simpler way to implement VTL connections uses a site code,consisting of one or more digits that a user must dial to specify the sitethat is being called. This approach requires no restriction on thetelephone extension ranges, but does require the caller to dial the sitecode digits as well as the extension.WANChicagoNBX SystemExtensions1000 – 1999AtlantaNBX SystemExtensions2000 – 2999DallasNBX SystemExtensions3000 – 3999
