As recently as 2005, voice revenues represented 73 percent of total revenues. By 2013, voice had dropped to just 43 percent of total revenues.

In a US telecommunications services industry that exceeded $500 billion in annual revenues in 2013, voice services—both wireline and wireless—represented about a third of industry revenues.

Insight Research projects that US telecommunications service revenues will continue to grow over the forecast period from 2013 to 2018.

But voice revenue will continue to decline at -4.81 percent compound annual growth rate from $163 billion in 2013 to $127 billion in 2018, Insight Research predicts.

Mobile voice—which peaked at $118 billion in 2008—will decline at -3.82 percent CAGR to $84 billion in 2018. Fixed network voice will drop even faster, at a negative 6.56 percent CAGR from $61 billion in 2013 to $44 billion in 2018.

Voice lines in service obviously will mirror those declines.

The US fixed network voice installed base peaked at 192 million access lines in 2000.

In following decade, despite nominal population growth, close to 80 million  access lines or more than 40 percent of wireline network lines were disconnected, replaced largely by mobile phones, Internet-based phone services or a shift to other communication apps, Insight Research says.

In 2002, the global telecommunications industry reached a crossover point at which the number of mobile service subscribers surpassed those of fixed telephone networks.

At the close of 2002, there were 1.2 billion mobile customers around the world, compared with 1.1 billion fixed telephone lines.

The situation in the United States was similar, as U.S. mobile call volumes doubled from 2005 to 2011, rising from 1,130 billion minutes of use to 2,296 billion minutes of use, while wireline volumes declined from 1,136 billion to 512 billion, dropping by half.

Those are daunting challenges for observers who argue that service providers actually can do very much to protect and then grow voice revenues. Some might argue that, no matter what service providers do, voice revenues will drop.