Editing Voice over IP (section)

==PSTN integration==
{{More citations needed section|date=November 2019}}

A VoIP media gateway controller (aka [[Class-5 telephone switch|Class 5]] Softswitch) works in cooperation with a media gateway (aka IP Business Gateway) and connects the digital media stream, so as to complete the path for voice and data. Gateways include interfaces for connecting to standard PSTN networks. Ethernet interfaces are also included in the modern systems which are specially designed to link calls that are passed via VoIP.<ref>{{cite web |url=http://www.ixc.ua/importance-of-softswitch-voip-technology-properly |archive-url=https://web.archive.org/web/20121111022027/http://www.ixc.ua/importance-of-softswitch-voip-technology-properly |archive-date=November 11, 2012 |title=Importance of Softswitch VoIP Technology |publisher=ixc.ua |date=May 20, 2011 |access-date=October 4, 2012 |url-status=bot: unknown }}</ref>

[[E.164]] is a global numbering standard for both the PSTN and [[public land mobile network]] (PLMN). Most VoIP implementations support [[E.164]] to allow calls to be routed to and from VoIP subscribers and the PSTN/PLMN.<ref>{{cite web|url=http://www.packetizer.com/rfc/rfc3824/|title=RFC 3824– Using E.164 numbers with the Session Initiation Protocol (SIP)|publisher=The Internet Society|date=June 1, 2004|access-date=January 21, 2009 |last1=Peterson |first1=Jon |last2=Liu |first2=Hong |last3=Campbell |first3=Ben |last4=Yu |first4=James }}</ref> VoIP implementations can also allow other identification techniques to be used. For example, [[Skype Technologies|Skype]] allows subscribers to choose ''Skype names'' (usernames)<ref>{{cite web|url=http://www.skype.com/help/guides/createskypename_windows/|title=Create a Skype Name|publisher=Skype|access-date=January 21, 2009}}</ref> whereas SIP implementations can use [[Uniform Resource Identifier]] (URIs) similar to [[email address]]es.<ref>{{cite web|url=http://www.packetizer.com/rfc/rfc3969/|title=RFC 3969– The Internet Assigned Number Authority (IANA) Uniform Resource Identifier (URI) Parameter Registry for the Session Initiation Protocol (SIP)|publisher=The Internet Society|date=December 1, 2004|access-date=January 21, 2009 |last1=Camarillo |first1=Gonzalo }}</ref> Often VoIP implementations employ methods of translating non-E.164 identifiers to E.164 numbers and vice versa, such as the Skype-In service provided by Skype<ref>{{cite web|url=http://www.skype.com/allfeatures/onlinenumber/|title=Your personal online number|publisher=Skype|access-date=January 21, 2009}}</ref> and the [[E.164 number to URI mapping]] (ENUM) service in IMS and SIP.<ref>{{cite web|url=http://ipcommunications.tmcnet.com/hot-topics/MCP/articles/1311-application-level-network-interoperability-the-evolution-ims.htm|title=Application-level Network Interoperability and the Evolution of IMS|publisher=TMCnet.com|date=May 24, 2006|access-date=January 21, 2009}}</ref>

Echo can also be an issue for PSTN integration.<ref>{{cite book|author=Jeff Riddel|title=Packetcable Implementation|url=https://books.google.com/books?id=8CNBbrxytcAC&pg=PA557|year=2007|publisher=Cisco Press|isbn=978-1-58705-181-4|page=557}}</ref> Common causes of echo include [[Impedance matching|impedance mismatches]] in analog circuitry and an acoustic path from the receive to transmit signal at the receiving end.

===Number portability===
[[Local number portability]] (LNP) and [[mobile number portability]] (MNP) also impact VoIP business. Number portability is a service that allows a subscriber to select a new telephone carrier without requiring a new number to be issued. Typically, it is the responsibility of the former carrier to "map" the old number to the undisclosed number assigned by the new carrier. This is achieved by maintaining a database of numbers. A dialed number is initially received by the original carrier and quickly rerouted to the new carrier. Multiple porting references must be maintained even if the subscriber returns to the original carrier. The [[Federal Communications Commission]] (FCC) mandates carrier compliance with these consumer-protection stipulations. In November 2007, the FCC in the United States released an order extending number portability obligations to interconnected VoIP providers and carriers that support VoIP providers.<ref>{{cite web|url=http://www.fcc.gov/cgb/consumerfacts/numbport.html|title=Keeping your telephone number when you change your service provider|work=FCC|access-date=January 20, 2009|archive-date=December 12, 2009|archive-url=https://web.archive.org/web/20091212070455/http://www.fcc.gov/cgb/consumerfacts/numbport.html|url-status=dead}}</ref>

A voice call originating in the VoIP environment also faces [[least-cost routing]] (LCR) challenges to reach its destination if the number is routed to a mobile phone number on a traditional mobile carrier. LCR is based on checking the destination of each telephone call as it is made, and then sending the call via the network that will cost the customer the least. This rating is subject to some debate given the complexity of call routing created by number portability. With MNP in place, LCR providers can no longer rely on using the network root prefix to determine how to route a call. Instead, they must now determine the actual network of every number before routing the call.<ref>{{Cite web |date=2006-11-13 |title=TelePassport takes the sting out of MNP |url=https://www.itweb.co.za/content/5yONP7EKaWJqXWrb |access-date=2022-07-19 |website=ITWeb |language=en |url-status=live |archive-url= https://web.archive.org/web/20220719090004/https://www.itweb.co.za/content/5yONP7EKaWJqXWrb |archive-date=2022-07-19 }}</ref>

Therefore, VoIP solutions also need to handle MNP when routing a voice call. In countries without a central database, like the UK, it may be necessary to query the mobile network about which home network a mobile phone number belongs to. As the popularity of VoIP increases in the enterprise markets because of LCR options, VoIP needs to provide a certain level of reliability when handling calls.

===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>