Some devices, like Treos and full-size BlackBerrys, opt for the larger size and the keyboard, while others assume you’ll peck out email or Web addresses on a phone keypad.

I’ve been testing a new $295 smart phone from Helio, an upstart company based in Los Angeles, that provides an elegant solution to this design problem. It’s called the Helio Ocean and it can look like either a standard voice phone or a keyboard-equipped email and Web device, depending on which way you open its unusual two-way sliding mechanism.

The Ocean also has some very nice software touches to complement this clever hardware design.

It isn’t as slender as some of its competitors and it has a few downsides, but the Ocean is an innovative, thoughtfully designed smart phone that advances the state of the art. It goes on sale over the next week or so at helio.com and in some retail stores later this month.

Helio not only designed the Ocean, but the phone works on Helio’s own cellphone service, which runs on Sprint’s network at broadband speeds. It lacks Wi-Fi wireless networking.

Plans with unlimited data access range from $65 to $135 a month, depending on the number of voice minutes. For $145 a month, you can get unlimited data and minutes.

When closed, the Ocean is just a roomy screen with some buttons at the top and bottom of a black rectangular body with rounded corners. If you hold it vertically with the screen in portrait mode and slide the screen up, a standard phone keypad is revealed that you can hold to your ear when making calls, just as on a standard voice phone.

If you turn the Ocean horizontally so its screen is in landscape mode and slide it up, a full typing keyboard is revealed that puts you in a comfortable position to compose messages and surf the Web. All the main functions are available in both modes and you can use a headset in either mode.

The software is smart, too. When you physically switch the Ocean from one orientation to the other, the screen display switches between portrait and landscape modes automatically. If you answer the phone while in horizontal landscape mode and you aren’t using a headset, the call automatically is placed in speakerphone mode, because it would be clumsy to hold the Ocean to your ear in that configuration. If you slide the keyboard closed and switch to vertical mode, the call continues and the speakerphone is automatically turned off.

When closed, the Ocean looks a bit stubby but it’s actually slightly smaller in every dimension than a Treo 700 — although it gets larger when the keyboard is exposed.

Voice calls were clear and crisp, and the speakerphone function worked well. It was easy to add numbers to the address book.

From the main screen, you can just start typing anything and the Ocean will either search your address book for the characters you type or initiate a Web search. Web-search results are presented in multiple search engines, including Google and Yahoo, Amazon and Wikipedia, which are arrayed in tabs.

I found the keyboard easy to use and was able to attain good accuracy on it while typing with my thumbs. However, the space between the top row of keys and the bottom edge of the screen is a bit cramped; it took some adjustment to thumb-type well on that row of keys.

The Ocean comes preconfigured for all the major consumer email services, including Yahoo, AOL, Hotmail and Gmail. You can manually add others. The Ocean also supports Microsoft Exchange email.

Unlike a Treo or a Windows Mobile phone, the Ocean doesn’t allow editing of Microsoft Office documents. You can only view them as text files and, in my tests, even that didn’t work. Helio plans new software that would allow the documents to display properly.

Similarly, the Ocean works out of the box with a variety of instant messaging services, including AIM, Yahoo and Windows Live Messenger. It also has multiple media functions. It sports a two-megapixel camera with flash and decent built-in software for displaying photos, and playing music and videos. It also accepts memory cards for expanded storage.

You can simply plug the Ocean into a Windows or Macintosh computer and drag photos, songs and videos onto its internal memory or memory card, if they are in a supported format. But in my tests of this, some photos wouldn’t display properly and none of the album art showed up in my MP3 songs. Helio says the album art will show up if you use synchronization software on Windows, instead of simply dragging the files over.

Despite some limitations, the Helio Ocean is an impressive device that’s fun to use.

• Email me at mossberg@wsj.com. Find all my columns and videos online free at the new All Things Digital Web site, http://walt.allthingsd.com.