When beginning a Lync Server 2013 voice deployment, one of the first steps is to determine the dial plan. The dial plan defines how many digits are used in each site, whether site prefixes are used, and any number translations required from site to site. Dial-plan objects in Lync Server 2013 contain a collection of normalization rules that are associated with a site or pool, or that are directly assigned to user accounts.
In many cases an organization’s existing dial plan can simply be reused and there is no requirement to assign new telephone numbers to users as they are migrated to Lync. This can be accomplished with some form of PBX integration, either via direct SIP trunk or through a media gateway.