Though it is understandable why telecom executives would prefer a steady, predictable, slow decline of voice revenues, there are signs that earlier forecasts were too sanguine. As recently as 2007, for example, International Telecommunications Union forecasts suggested stable, flat voice revenues. ITU voice revenue forecast

In fact, total telecom revenue grew by seven percent a year to eight percent a year, on average, from 1991-2004. The highest growth was in mobile telephony, with domestic and international revenues declining slightly. Voice services were, in 2007, seen as roughly stable, representing about 85 percent of total service provider revenue in 1991, and 82 percent in 2004, for example.

Even in the highly developed countries of the Organization for Economic Cooperation and Development region, voice still accounted for 79 percent of total telecom revenues in 2007.

But the ITU also argued that voice might “be given away for free over a ten-year time horizon.”A contribution decline from “80 percent of revenues” to “zero” over anything like a 10-year time frame would be a challenge for any executive at any company, in any industry.

But some of us would argue that a decline from 50 percent of total revenues to 30 percent of revenues not only is possible, but already has been accomplished, at least in the U.S. market, where telecom service providers faced a decline of long distance revenue from 50 percent to 25 percent in just 10 years between 1997 and 2007. Past transitions in telecom

Recent reports from service providers with fixed and mobile voice operations suggest the rate of decline might be accelerating. Belgacom voice revenue decelerates

Likewise, KPN (News - Alert) saw voice revenue declines it attributed to consumer use of rival “over the top” alternatives. KPN sees voice revenue drop

Looking at global telecom revenue sources over the next couple of years, some basic trends can be seen. Fixed network services other than broadband continue to decline.

Wireless revenues continue to grow, as does broadband access revenue.

Keep in mind that these are global, aggregate numbers, buoyed by huge broadband and especially wireless growth in developing regions. The patterns can be quite distinct in specific national markets.

In the U.S. market, it is conceivable that video and content revenues could be a somewhat significant factor over a decade-long time frame. Wireless growth will be highly susceptible to broadband and data services growth, balanced by a certain amount of "harvesting" of mobile voice revenue, which will decline, relative to broadband, over the decade.

It is worth noting that voice revenue trends have been through two fundamental cycles, with a third on the way. At one time, international long distance was the highest-margin product, followed by domestic long distance. That changed fundamentally between 1997 and 2007. Over that 10-year period, long distance, which represented nearly half of all revenue, was displaced by mobile voice services.

Since about 2000, fixed voice lines and revenue have been steadily declining, at least in the telecom service provider segment, with the cable segment able to grow the role of voice in overall revenue.

In the third change, mobile voice will follow a trend similar to that of long distance.

In each of the shifts already occurring, several things happened. Prices and profit margins steadily were compressed. And new competitors picked up significant share of the remaining business. In each of the three periods, the product has changed.

Between about 1997 and 2007, "long distance" became loosely coupled with local calling and local access. Long distance increasingly could be consumed "over the top," using prepaid calling cards or separate providers for "long distance" on a local line, for example.

Between 2000 and 2009, it became possible to use mobile phones to similarly displace both local and long distance calling, as well as to substitute mobility for fixed voice, while both over-the-top and IP-based calling options became available.Over the next 10 years, both voice itself and long distance calling in the mobile and fixed realms likewise will be increasingly disaggregated and amenable to "over the top" consumption.The ITU prediction of voice essentially being given away might not materialize fully, or within a decade, but it is the trend.