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A VoIP phone uses voice over IP (VoIP) technologies allowing telephone calls to be made over an IP network such as the Internet instead of the ordinary PSTN system. Calls can traverse the Internet, or a private IP network such as that of a company. The phones use control protocols such as Session Initiation Protocol (SIP), Skinny Client Control Protocol (SCCP) or one of various proprietary protocols such as that used by Skype.
Softphones, Smartphones, USB Hard Phones, PoE Hard Phones 
VoIP phones can be simple software-based softphones or purpose-built hardware devices that appear much like an ordinary telephone or a cordless phone. Ordinary PSTN phones are used as VoIP phones with analog telephone adapters (ATA).
Two combined signal-and-power wired cable interfaces are in common use to communicate between computer networks (or computers) and physically separate VoIP phones: USB and Power over Ethernet. The latter is preferred in industry such as large scale call centers or PBX replacement applications, because PoE has the following advantages over USB:
- Much greater and more flexible power allocation: PoE switches typically support 4, 8, 16, 24, 48 or more ports each, with each device drawing up to 30W. By contrast, a USB 3.0 only supports up to 4.5W devices, insufficient for cordless, conference, videoconferencing and other common-use devices.
- Much greater cord range. While DC power diminishes rapidly over long cables, and is inefficient to supply further than 3M for high-draw devices, the low power draw of modern indoor radios permits full use of cat5 Ethernet cables of up to 100m length. By contrast USB supports under 3m and with extensions and boosters perhaps 10m maximum.
- Single connector: PoE relies on the single uniform rj-45 connector, a close relative of the proven RJ-11 connector used for analog phones. By contrast there are six USB connectors and each change to the USB standard so far has involved at least one new type of connector.
- Ethernet protocol: Scalable from its original 1 megabit to modern 100 gigabit applications, copper Ethernet has no peer for scalability. It makes effective use of many different cable types (especially cat3 and coax) and has robust wireless equivalents (802.11/WiFi) when power and wired security is not required.
For these reasons, USB and softphone PC applications are considered transitional by many industrial users and makers of larger telephone switches, used only to build markets for VoIP that will eventually shift over to the more robust PoE technology shipped by Cisco, Siemens, Alcatel and other large switch makers.
A VoIP phone or application may have many features an analog phone doesn't support, such as e-mail-like IDs for contacts that may be easier to remember than names or phone numbers, or easy sharing of contact lists among multiple accounts. Generally the features of VoIP phones follow those of Skype, Google Voice and other PC-based phone services, which have richer feature sets but (because they rely on mainstream operating systems' IP support) latency-related audio problems.
A competing view is that as mainstream operating systems become better at voice applications with appropriate Quality of Service (QoS) guarantees and 5G handoff (IEEE 802.21 etc.) becomes available from outdoor wireless carriers, netbooks and smartphones will simply become the dominant interfaces. iPhone, Android and the QNX OS used in 2012-and-later BlackBerry phones are generally capable of VoIP performance even on small battery-charged devices. They also typically support the USB but not Ethernet or Power over Ethernet interfaces, at least as of late 2011. According to this view, the smartphone becomes the dominant VoIP phone because it works both indoors and outdoors and shifts base stations/protocols easily to trade off access costs and call clarity and other factors personal to the user, and the PoE/USB VoIP phone is thus the transitional device.
Elements of a VoIP phone 
- Physical Hardware
- DNS client
- STUN client
- DHCP client (not commonly used)
- Signalling stack (SIP, H.323, Skinny, Skype, or others)
- RTP stack
- User interface
Hardware of a standalone VoIP phone 
The overall hardware may look like a telephone or mobile phone. A VoIP phone has the following hardware components.
- Speaker/ear phone and microphone
- Key pad/touch pad to enter phone number and text (not used for ATAs).
- Display hardware to feedback user input and show caller-id/messages (not used for ATAs).
- General purpose processor (GPP) to process application messages.
- A voice engine or a digital signal processor (DSP) to process RTP messages. Some IC manufacturers provides GPP and DSP in single chip.
- ADC and DAC converters: To convert voice to digital data and vice versa.
- Ethernet or wireless network hardware to send and receive messages on data network.
- Most IP phones have an Ethernet port for a PC. This allows your computer and phone to share one data connection, routed through the phone and to the computer.
- Power source might be a battery or AC source. Some VoIP phones receive electricity from Power over Ethernet.
- Some VoIP phones include an RJ-11 port to connect the phone to the PSTN.
For wireless VoIP phones
Other devices 
Gateway devices 
Analog telephony adapters are connected to the internet or Local area network using an Ethernet port and have sockets to connect one or more PSTN phones. Such devices are sent out to customers who sign up with various commercial VoIP providers allowing them to continue using their existing PSTN based telephones.
Another type of gateway device acts as a simple GSM base station and regular mobile phones can connect to this and make VoIP calls. While a license is required to run one of these in most countries these can be useful on ships or remote areas where a low-powered gateway transmitting on unused frequencies is likely to go unnoticed.
Stun client 
A STUN (Session Traversal Utilities for NAT) client is used on some SIP-based VoIP phones as firewalls on network interface sometimes block SIP/RTP packets. Some special mechanism is required in this case to enable routing of SIP packets from one network to other. STUN is used in some of the sip phones to enable the SIP/RTP packets to cross boundaries of two different IP networks. A packet becomes unroutable between two sip elements if one of the networks uses private IP address range and other is in public IP address range. Stun is a mechanism to enable this border traversal. There are alternate mechanisms for traversal of NAT, STUN is just one of them. STUN or any other NAT traversal mechanism is not required when the two SIP phones connecting are routable from each other and no firewall exists in between.
DHCP client 
A DHCP client may be used to configure the TCP/IP parameters and server details if a network segment uses dynamic IP address configuration. The DHCP client then provides central and automatic management of VoIP phones configuration.
Common features of VoIP phones 
- Caller ID
- Encrypted communications
- Dialing using name/ID (differs from speed dial in that no number is stored on the client)
- Locally stored and network-based directories
- Conference and multiparty call
- Call park
- Call transfer and call hold
- Preserving user name/ number when choosing a different service provider (not widely supported).
- Applications like weather report, Attendance in school and offices, Live news etc.
Disadvantages of VoIP phones 
- Requires Internet access to make calls outside the local area network (LAN) unless a compatible local private branch exchange (PBX) is available to handle calls to and from outside lines.
- VoIP phones and the routers depend on mains electricity for power, unlike PSTN phones, which are supplied with power from the telephone exchange. However, this can be mitigated by installing a UPS. The Power over Ethernet interface simplifies this immensely since power can be "injected" at any connector (especially in passive mode where all devices are drawing the same voltage) or at the router. This is a major reason the dominant call center and PBX VoIP systems rely on PoE exclusively)
- IP networks, particularly residential Internet connections are easily congested. This can cause poorer voice quality or the call to be dropped completely. As commercial grade routers begin to incorporate "managed" or "carrier" QoS features, this is less of an issue, but such features typically require expert configuration.
- VoIP phones, like other network devices can be subjected to denial-of-service attacks as well as other attacks especially if the device is given a public IP address; This is especially significant as an issue with wireless devices using 802.11 protocols
- Due to the latency induced by protocol overhead and other factors they do not work as well on satellite Internet, analog cell ("edge" networks) and other high-latency Internet connections. Extremely latency sensitive applications (music, remote device control) as of 2012 simply cannot exploit VoIP protocols.
- Proprietary vendors such as Skype and Google Voice focus on improving call quality between their own users to grow their user base, which to some degree competes and conflicts with the goal of better connections from Skype to Google Voice, or from either to the existing PSTN and cellular networks. The best codecs tend to be proprietary and not licensed to competitors, retarding the growth of the industry and causing incompatibility.
See also 
|Wikimedia Commons has media related to: IP Phone|
- List of SIP software
- IP Multimedia Subsystem (IMS)
- Media phone
- Mobile VoIP
- Cloud Telephone systems