The electrical interface is perhaps the easiest to achieve. On the analog side, the phone plugs into a standard RJ-11 jack like you would find on a wall socket. This delivers the standard -48 V DC to power the handset along with the dial tone, AC ringing, and Caller ID signals when needed. It has to detect standard DTMF (Dual Tone Multi-Frequency) touch-tone waveforms from the keypad and convert them to digital format for IP signaling using Session Initiation Protocol, the standard for VoIP.
How does the analog voice become digital and back to analog again? That's the function of the Codec or Coder/Decoder. The standard in use since digital telephony was invented is G.711. But there are other codecs that must also supported. These may include G.723, G.729, G.728 and G.726.
On the network side, The ATA has to terminate a standard Ethernet connection, usually 10/100 Mbps through a RJ-45 connector labeled as WAN. Many newer ATAs take this a step further by including a second Ethernet port labeled LAN. This is for connection to a PC in lieu of connecting the PC directly to DSL, Cable Modem or T1 WAN. The ATA performs the function of an Ethernet switch or router. This is not just to save the cost of a separate device so that PC and telephone can share the same broadband connection. When a router is included in the ATA, it then has the ability to ensure quality of service by giving voice packets priority over computer data packets so that using the PC doesn't interrupt telephone calls.
Maintaining voice quality while minimizing bandwidth is also the function of signal processing within the ATA. Some common processes include line echo cancellation and dynamic jitter buffer that compensates for variations in Internet line speed. PLC or Packet Loss Concealment deals with missing or out of sequence packets. Either the packet is replaced with a zero packet, a copy of an already received packet, or an interpolation between the preceding and following packets. VAD or Voice Activity Detection senses when you are talking and stops generating packets while you are listening to reduce bandwidth needs. Because digital lines have no traditional analog "hiss", people sometimes equate silence with a lost connection. CNG or Comfort Noise Generation recreates that low level background noise to indicate that the connection between phones is still in place.
All of these functions have been integrated into a package you can hold in your hand, in the HandyTone 502 ATA that is provided at no cost when you order PhonePower VoIP telephone service for residential and small business use. You can use the analog phone you have now, including cordless phones with multiple handsets.
Other VoIP service providers have also gone the route of providing their customers with complimentary ATAs to make the transition from traditional analog to digital VoIP as easy as possible.

To get a bead on Cisco's thinking, we first need to take a look at the video camera that has taken the camcorder market by storm. The original Flip Video camcorder was innovative in the same sense as Apple's iPod. There were music players before the iPod and camcorders long before the Flip. But Pure Digital created a product for capturing video that mirrors the clever design that Apple applied to the iPod. Both products are pocket-size, offer high quality performance, are trivially easy to use, easily connect to personal computers, and have internal power sources compatible with their expected usage.
Now don't worry. This will only take a few minutes and you can be on your way. First, you'll need to find that old cell phone you tossed somewhere when you got your brand new one. Did you know that there are something like 100 million cell phones set aside each year? If you piled them all up, you'd find enough gold inside to make a respectable pot at the end of the rainbow. Before the leprechauns get wind of this, you better retrieve that unused phone and see how you can profit from it.
Take this phone out of the box and it looks like many of the popular touchscreen phones that users are snatching up right and left. But look again and you'll see an extra accessory that comes with no other phone. It's a detachable QWERTY keyboard. This is an actual physical keyboard about the same size as the phone, but mounted in a case that can be attached or detached from the main phone body. 