From their user interface to how they work, WiFi phones are a lot like cell phones . Like a basic cell phone, a WiFi phone has a printed circuit board (PCB) that connects:
Both types of phones also send and receive signals as radio waves. The difference is that WiFi phones use different frequencies than cellular phones do. Cell phones use 824-MHz to 894-MHz frequency bands. WiFi phones that use the 802.11b or 802.11g standards transmit at 2.4 GHz. Phones that use the 802.11a standard transmit at 5 GHz.
When you make a call on a WiFi phone, you dial the number of the person you want to call, just like you would with a cell phone. If you're calling another VoIP user, you may enter a VoIP address instead of a phone number, depending on the service provider's requirements.
The phone translates the number you dial into packets of data. It uses radio waves to transmit the packets to a wireless receiver. The receiver passes the information over the Internet to the call processor like an ordinary VoIP call. When you begin your conversation, the phone transmits your voice in packets of data as well. Your voice travels just like it does in a VoIP call, although the specifics can differ from one provider to another.
Many of these phones use a specific WiFi service or network. For example, Netgear makes a WiFi phone for Skype service, and UTStarcom makes a WiFi phone for Vonage service. In order to use either of these phones, you need to have an account with that provider, just like you need a service plan from a cell phone provider to use a particular phone. Either you or your service provider will configure the phone to work within the network.
Other phones work with a protocol rather than a particular network or provider. For example, ZyXEL and Linksys both make WiFi phones that use Session Initiation Protocol (SIP). SIP is one of several VoIP protocols -- you can find out more about them at Protocols.com. After setting up a SIP phone, you can make calls simply by entering the other party's SIP address and pressing "Send."
SIP is an open protocol. Anyone can use it, and anyone who has a SIP address can contact anyone else who has one for free. SIP is a standard protocol for handling voice data, and many VoIP providers use it, even though they maintain a closed network. It can also control other forms of communication, like instant messaging and video conferencing.
Eventually, SIP will probably make communications tools interoperable. In other words, your computer, phone, PDA and other communications tools could all share the same address book and communicate with one another.