THE COOLEST thing about The T-Mobile Pocket PC Phone is what happens when it rings.

The other cell phones on the block go beep-beep-beep or play an electronic bagpipe version of the "William Tell Overture," but the phone I've been testing really rings. As in "Brrrring, brrrrring." Just like an old-fashioned phone, if you're old enough to remember one.

Retro ring tone aside, this $500 bleeding-edge gadget is a digital tour-de-force that combines a Personal Digital Assistant (PDA) with a cell phone and wireless Internet appliance.

It also works, but that doesn't mean I'd buy one, even though I'm a gadget lover. Although its three components performed well individually and even cooperated, the T-Mobile is a good example of what happens when you try to pack too many goodies into one lunchbox.

Still, it's worth looking at as a harbinger. Cell phone and computer makers are betting on multifunction gadgets like this one to get consumers excited enough to revive sales in both markets.

The problem is finding the right combination of form and function. Since the Palm Pilot burst on the scene in 1996, some 29.3 million PDAs have been sold worldwide, according to Gartner Research. Compared to 1.1 billion current cell phone users, PDA enthusiasts are a drop in the bucket. So if you were a designer, where would you start with a gadget that does it all?

You can start with a cell phone, which is familiar and small enough to carry comfortably but doesn't have much screen real estate or a keyboard for entering text. Or, you can choose a pager-style device, with a tiny, thumb-operated QWERTY keyboard. It's easy to carry, OK for e-mail and short messaging, but doesn't lend itself to voice.

Finally, you can try a PDA, which has a bigger screen and can even recognize handwriting. But a PDA isn't the kind of gadget you'd want to tuck in your pocket for a camping trip.

"To some extent, all of these converge devices are compromises," says Todd Kort, a principal analyst for Gartner who follows the PDA market. "I haven't seen any of them yet that I think are outstanding."

T-Mobile (the new name for VoiceStream, the U.S. cell phone and wireless data subsidiary of Germany's Deutsche Telekom) bet on a handheld computer. It's the first that uses a specialized version of Microsoft Windows for Pocket PCs, which we'll call "Microsoft Phone." Compaq, Toshiba and AudioVox plan similar devices.

At 5 inches tall by 2.8 inches wide, with a backlit, 3.5-inch screen that displays 4,096 colors, the T-Mobile is a pleasure to work with when you're reading, writing or browsing. Unfortunately, it also weighs 7 ounces, enough to put a real strain on any pocket. The average cell phone weighs only 3 to 4 ounces.

I like PDAs running Microsoft's handheld OS. They're bigger and more expensive than handhelds from Palm, Handspring, Sony and others who license the simple and friendly Palm OS. But PocketPCs can also do more out of the box, thanks to tiny versions of Microsoft Word, Excel, Media Player and other software. For example, the T-Mobile can make a ring tone out of any standard wave file - my "Brrrring" is actually a recording of an old-fashioned phone.

Microsoft Phone adds an on-screen dialer to the Windows Pocket PC interface, along with a feature that dials a number directly from the Outlook contact manager, or even from a Web page if it's formatted properly.

You can dial the phone by tapping the pressure-sensitive screen with your finger as well the stylus, or use a circular scroll button mounted at the bottom of the cover. Even so, the device is hard to manage one-handed, which makes it problematic in a car.

Likewise, the T-Mobile is too wide to talk on comfortably on for more than a few minutes - your hand gets tired. You can switch to speaker-phone mode, but that eliminates privacy. Or you can also use a bundled, earbud headset. But I found the earpieces too big for comfort, and finding a replacement isn't easy - T-mobile uses a plug that only few manufacturers have adopted.

That said, I had no trouble calling from my home, which is normally cell phone hell. This surprised me, since T-Mobile is the smallest of the major providers in the U.S. It's also the only one that relies solely on the GSM phone standard, which is dominant in the rest of the world but less common here. T-Mobile says it covers in 45 of the 50 largest U.S. markets, but if you travel outside major population centers, you might want to look elsewhere for a cellular service.

On the Internet side, I had no trouble retrieving e-mail from my home and work accounts or even browsing the Web, for that matter, which absosutely astounded me. But most Web pages won't fit into the display unless they're formatted for use by devices that adhere to the Wireless Application Protocol (WAP).

To enter information, Microsoft gives you the choice of tapping an on-screen keyboard, which is agonizing, or using two forms of handwriting recognition. The first requires that you write one character at a time in a pad at the bottom of the screen; the second tries to decipher your regular scrawl.

Character recognition works well, but it's still awkward for anything but short messages. We'll call the handwriting recognition a work in progress: you'll spend more time cleaning up the mess than it's worth.

There's also a matter of cost. To use all the phone's features, you'll need separate voice and data accounts. T-Mobile offers voice plans similar to its competitors', but its data charges are based on megabytes transferred, not minutes. Plans start at $19.99 per month for 5 megabytes of data, which the company says is enough for 90 Web pages or 850 e-mails That can add up, but may well be worth it if you want to keep in touch without lugging a laptop.

Another issue is battery life. T-mobile claims 4 hours of talk and 100 hours of standby use for the phone, and 12 hours of battery life for the PDA. But that drops quickly if you use the backlight. So, using the PDA intensively for several hours might leave you with a dead phone.

My take: if you want an organizer with wireless e-mail and Internet connectivity, the T-Mobile is a great gadget that's priced about the same as older Pocket PCs without wireless connectivity. I'd forget about the T-Mobile as a phone, though. It's just too clunky.

If you're mainly interested in talk and e-mail, take a look at Handspring's new Treo 300, which runs on the Palm operating system. It's has a smaller screen than the T-mobile, but it's more compact, lighter and includes a QWERTY keyboard, as well as a stylus.