T-Mobile's entry into the landline voice-over-IP business last week was the latest accelerant of the trend toward voice as an application owned predominantly by mobile operators.

Though wireless substitution has been under way for years, we've only recently seen key milestones suggesting the extent to which assumptions about the triple play — voice, video and data — have been flawed, in that voice, without mobility, is merely a subset of data and increasingly is being treated as such.

AT&T and Verizon — the only companies truly capable of delivering the ultimate bundle of wireless and wireline voice, video and data — are more and more positioning voice as a mobile service. This month, Verizon announced a discount for customers who take its double play of broadband and mobility without landline voice. The move wasn't surprising after AT&T cited a double play of wireless and stand-alone DSL — again, without fixed voice — as the main driver of its surprisingly high first-quarter broadband growth.

Meanwhile, as mobile operators such as T-Mobile join a host of over-the-top providers in effectively penetrating the landline voice market, it's becoming clear how difficult it is for landline carriers, in turn, to take on mobility. Cable companies, Embarq and Qwest Communications all have been frustrated in their efforts. Triple-play provider SureWest Communications gave up and got out.

Barely a month after Embarq announced it was exiting the mobile business, the company announced it was outsourcing its landline voice operations to Nokia Siemens Networks. Far from a coincidence, the move illustrates how even a wireline evangelist such as Embarq has recognized the unsustainable economics of fixed voice.

In March, Speakeasy, which offers bundled broadband services nationwide to small businesses as part of Best Buy, began offering what might be called “naked voice” — that is, VoIP over any broadband line, not just their own. But the main rationale for this business is to reach its customers' off-network locations and get a foot in the door so that when a competitor's T-1 contract expires, for example, Speakeasy can try to sell the whole pipe, which is the real endgame.

The more that providers of traditional voice service come to admit that they're playing a losing hand — despite all the quality built into that service, most consumers will happily trade a little quality for mobility and/or a big discount — the more they can take full advantage of their true strength in the market: bandwidth. Awaiting them are a potentially infinite number of services and applications that can be offered only through the thick pipes of a landline local loop. Carriers can get a better jump on those new markets the sooner they give up on voice.