HD voice generates frequencies at the high end of human speech that toll-quality voice cuts off. In practical terms this means being able to hear the difference between words like "pig" and "fig" without asking whether that's a p as in Peter or an f as in Frankenstein.

Wideband voice pushes up the top frequency range covered by the voice coder-decoders (codec) from about 4 kHz to something higher, typically 7 kHz, but it can go even higher to 14 kHz or 20 kHz. One consequence of this wider band is the need to sample speech more times per second in order to capture more of its subtleties. Rather than 8,000 times per second, sampling jumps to 48,000 times per second.

That means the VoIP phones supporting HD need more powerful processing and memory, both of which cost more, but with Moore's Law applied to the phones, the cost differential is narrowing. For example, Polycom's SoundPoint IP 501 phone with Power over Ethernet (PoE) and three lines but with no wideband support costs US$295. A roughly comparable SoundPoint IP 550 with PoE, four lines and HD support costs $369.

As introduces HD phone models, it is phasing out the equivalent narrowband phone, so eventually all its line will support wideband and remain backward compatible with narrowband, Rodman says.

Because the endpoints and the networks between the endpoints must both support HD voice in order for it to work, most wideband deployments are within corporations, not between them, says William Bumbernick, CEO of Alteva, hosted VoIP provider. "The ability to call a different company on a different carrier and completing an HD call is becoming more and more expected," he says, but "today, the chances are still pretty slim that that will happen."