Paul Farhi covers media for The Post.

Like a few million other people this past holiday season, we bought an iPhone 4S, with its much-hyped Siri feature. The vocal interface allows users to speak all kinds of commands into the phone (“What’s the weather in San Francisco?”) and get answers from a sultry-voiced robot/concierge.

We’ve used Siri to get directions, to make hands-free mobile calls and to fetch answers to trivia questions. Sometimes we just goof on Siri. “Siri, do you love me?” my daughter asked the other day. (Siri’s heartbreaking response: “I am not capable of love.”) Most ways you look at it, Siri is pretty magical.

But not in every way. Siri’s dirty little secret is that she’s a bandwidth guzzler, the digital equivalent of a 10-miles-per-gallon Hummer H1.

To make your wish her command, Siri floods your cell network with a stream of data; her responses require a similarly large flow in return. A study published this month by Arieso, an Atlanta firm that specializes in mobile networks, found that the Siri-equipped iPhone 4S uses twice as much data as does the plain old iPhone 4 and nearly three times as much as does the iPhone 3G. The new phone requires far more data than most other advanced smartphones, which are pretty data-intensive themselves, The Post has reported.

In all, Arieso says that the Siri-equipped iPhone 4S “appears to unleash data consumption behaviors that have no precedent.”

Under most circumstances, this would seem to be someone else’s problem. Cellphone contracts are “tiered” so that those who use a network more than others pay more for the privilege. You want to ask Siri silly questions? Go to town — but you (or, in this case, I) will get the bill at the end of the month. By the same logic, a customer who wants better service on an airline can pay for it by buying a first-class ticket. The marketplace provides.

Except on the data skyway, it’s not that simple. Cell and data networks are like any common resource; they have limits. And once they hit their limit, regardless of which group is using its share and then some, there’s no more to go around.

This means that Siri’s data-hogging ways are a problem for more than just those willing to foot the bill. As networks become congested, everyone’s service deteriorates. Private desire becomes a public issue. Calls are dropped or never completed; Internet access slows. First-class airline passengers don’t really compromise service for those in coach. But bandwidth hogs do.

The obvious response to this kind of traffic jam is to build more roads. Indeed, for years, cellphone companies and other network operators have expanded carrying capacity; they’ll undoubtedly continue to do so as long as demand is there.

But those kinds of infrastructure improvements aren’t limitless, either; no matter how many cell towers we throw up, sooner or later we’ll bump up against the rigid limits of the electromagnetic spectrum, the invisible frequencies over which all electronic communications move. And building new capacity isn’t cheap. Everyone — not just the first-class passengers — ends up paying for it. So prepare for higher cellphone bills.

And in the meantime? Prepare to sit and wait. That call to Grandma might not get through until the congestion clears.

Other alternatives might be less palatable, especially to anyone who wants immediate downloading gratification. We could stay off the grid or utilize fewer data-intensive functions. Or we could put some traffic cops on the beat to regulate our data demands and limit the traffic snarls and bottlenecks.

In any case, we’ll run out of capacity soon enough. The government’s top airwaves cop, Federal Communications Commission Chairman Julius Genachowski, has long warned about a looming “spectrum crunch.” If the United States can’t free up more bandwidth for mobile uses, more people than just cellphone users would be inconvenienced, he warns. The lack of new capacity, he says, would threaten U.S. jobs in the telecom industry and stifle technical progress.

Trouble is, like beachfront property, there is only so much spectrum to go around. All of the various parts of the airwaves are spoken for, whether auctioned by the government to high bidders over the past 15 years or given away back when radio and TV were the newest consumer technologies.

The only way to free up some now is to reshuffle the lineup, moving older users (say, over-the-air TV and radio stations and government agencies) to another part of the band in favor of the up-and-coming hot shots. Of course, that kind of change is disruptive. A massive political battle looms, pitting the haves against the want-mores.

What to do? Maybe we should ask the roadhog herself. Hey, Siri: Can we all learn to share?