Editing Voice over IP (section)

===Emergency calls===
A telephone connected to a [[land line]] has a direct relationship between a telephone number and a physical location, which is maintained by the telephone company and available to emergency responders via the national emergency response service centers in form of emergency subscriber lists. When an emergency call is received by a center the location is automatically determined from its databases and displayed on the operator console.

In IP telephony, no such direct link between location and communications end point exists. Even a provider having wired infrastructure, such as a DSL provider, may know only the approximate location of the device, based on the [[IP address]] allocated to the network router and the known service address. Some ISPs do not track the automatic assignment of IP addresses to customer equipment.<ref name=fcc>{{cite web|url=http://www.fcc.gov/cgb/consumerfacts/voip911.pdf |website=FCC |title=FCC Consumer Advisory VoIP and 911 Service|access-date=May 2, 2011 |url-status=dead |archive-url= https://web.archive.org/web/20100810193236/http://www.fcc.gov/cgb/consumerfacts/voip911.pdf |archive-date= Aug 10, 2010 }}</ref>

IP communication provides for device mobility. For example, a residential broadband connection may be used as a link to a [[virtual private network]] of a corporate entity, in which case the IP address being used for customer communications may belong to the enterprise, not the residential ISP. Such [[off-premises extension]]s may appear as part of an upstream IP PBX. On mobile devices, e.g., a 3G handset or USB wireless broadband adapter, the IP address has no relationship with any physical location known to the telephony service provider, since a mobile user could be anywhere in a region with network coverage, even roaming via another cellular company.

At the VoIP level, a phone or gateway may identify itself by its account credentials with a [[Session Initiation Protocol]] (SIP) registrar. In such cases, the [[Internet telephony service provider]] (ITSP) knows only that a particular user's equipment is active. Service providers often provide emergency response services by agreement with the user who registers a physical location and agrees that, if an emergency number is called from the IP device, emergency services are provided to that address only.

Such emergency services are provided by VoIP vendors in the United States by a system called [[Enhanced 911]] (E911), based on the [[Wireless Communications and Public Safety Act]]. The VoIP E911 emergency-calling system associates a physical address with the calling party's telephone number. All VoIP providers that provide access to the public switched telephone network are required to implement E911, a service for which the subscriber may be charged. "VoIP providers may not allow customers to opt-out of 911 service."<ref name=fcc/> The [[VoIP E911]] system is based on a static table lookup. Unlike in cellular phones, where the location of an E911 call can be traced using [[assisted GPS]] or other methods, the VoIP E911 information is accurate only if subscribers keep their emergency address information current.<ref>{{Cite web |last=Noworatzky |first=Daniel |title=Keep your VoIP E911 service in compliance with this checklist |url=https://info.teledynamics.com/blog/keep-your-voip-e911-service-in-compliance-with-this-checklist |access-date=2022-07-19 |website=TeleDynamics |date=Feb 6, 2019 |language=en-us}}</ref>