A phone number routed from the network to a specific phone in a PBX requires unique DID trunks or ISDN PRI “two-way DID” trunks. Blocks of DID numbers (typically 10 or 20) are purchased from the LEC or CLEC for use on the PBX. The number of DID numbers usually substantially exceeds the number of trunks in the system.

Dial Plans are strings of characters that convert dialed numbers into outbound dial strings. They can act as filters by allowing, disallowing, or modifying dialed numbers. When a dialed number matches a set Dial Plan on the device, the device will send the numbers outbound. Dial Plans can prevent calls to specific destinations like 411 and International Numbers or add an area code for 7-digit dialing. Dial Plans may look alike across manufacturers but can differ. Although most Dial Plans are based on MGCP, they may have modifications for different purposes.

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In the United States, there is a designated phone number for emergencies related to police, fire, or medical assistance. This number is assigned under the North American Numbering Plan (NANP), and is answered by a telephone operator or an emergency service dispatcher. They will then notify the appropriate emergency service to respond to the situation.