What’s a phone call worth these days?

A Verizon phone booth in a Washington subway station suggests one answer: 50 cents.

Another comes from Verizon’s cheapest landline service option, which charges 10.2 cents a call.

If you use a cell phone or subscribe to a voice-over-Internet- Protocol calling plan, the number shrinks to a vanishingly tiny fraction of your monthly bill: maybe a few pennies each time you dial out?

But if you use Google’s new, free phone-calling option (gmail.com/call), that figure drops to zero.

Last week, the Web giant announced that American users of its Gmail Web service could call numbers in the United States and Canada for free from within their browsers. Calls elsewhere cost less than many traditional long-distance domestic calls: You pay 2 cents a minute to call Ireland, Korea, Argentina and many other countries. (Google’s rates top out at 99 cents a minute for those calling the island nation of Nauru.)

The Skype Internet-calling service charges slightly more for international calls, but it also charges about 2 cents a minute for domestic calls.

Gmail’s rates should be familiar to users of Google’s free Google Voice service, a separate option that the Mountain View, Calif., company only opened to the public in June. But using Google Voice requires logging into a Web site or launching a program on your phone (if one is available for it) and, in some cases, waiting for Google to connect your call.

Calling from within Gmail, by contrast, requires nothing more than installing a small plug-in program (available for Windows XP or newer, Mac OS X 10.4 or newer and some versions of Linux) and logging into Gmail. Click the “Call phone” link to the left of your inbox, type in a number, click the big blue “Call” button and things proceed as if you had just finished spinning a Bell System phone’s rotary dial.

If you have a Google Voice account, the other person will see that number in their caller ID. You can also answer their calls from within Gmail; an incoming call will generate an alert in the bottom right corner of the Gmail window, which you can click to answer.

If you don’t have a Voice account, the other party will see a special number Google has set up, (760) 705-8888. Calls to that yield a message advertising the new Gmail feature.

I’ve used Google’s voice calling from the Safari and Firefox browsers on a Mac, as well as copies of Firefox running in Windows XP and Ubuntu Linux. Everything sounded fine to me, although one co-worker commented that my voice sounded as if I were underwater.

Google says more than 1 mil-lion calls were placed through Gmail in its first 24 hours. It’s unclear how many of them consisted of people dialing their cell phones, recording a voice-mail message to the effect of, “Hi, this is me calling through Gmail,” and then hanging up.

Google won’t say whether domestic calling will remain free, but it has structured this service and Google Voice to stay afloat based on the profit generated by international calling.

Unlike Gmail itself and many other Google applications, advertising doesn’t factor into this — a detail that its privacy policy ought to spell out but does not.

In other words, as spokesman Randall Sarafa wrote in a recent e-mail, “Google absolutely does not record or listen in on phone conversations.”

Placing a call through a Web browser may not be for everybody. But the ability to do this could change how even the tech-averse make and pay for phone calls.

Think about what Gmail did for e-mail: By offering effectively unlimited storage and inviting users to hold on to their old e-mailforever— after Microsoft and Yahoo had steadily cut back on the storage offered to users of their free Web-mail services — Google pounded the market price for each message all the way down to $0.00.

Phone use has been edging in that direction for a while, coaxed along by steadily expanding blocks of unmetered domestic calling time on wireless plans and the growing allotments of “VoIP” services such as Skype and Vonage. Google’s latest move can only accelerate that trend. Incumbent carriers will have to respond accordingly.