The one big thing Google Voice doesn't offer, besides automated telemarketer taunting, is free voice-over-internet phone calling through your cellphone. On an Android phone, however, you can use the Gizmo5 service, Google Voice, and a free application to call anyone for free.

A free, open-source, and unofficial Android app, Guava, gives any Android phone the ability to make and take calls over Gizmo5's VoIP service, connected through a Google Voice phone number. It works over Wi-Fi, 3G, or, for the daring, EDGE. You probably won't want to use Guava as your primary phone call manager, as the call quality varies with your connection and really works best over Wi-Fi. That said, if you're running up against your minute allotment, or find yourself in a basement-like spot with decent Wi-Fi but really bad cell coverage, Guava is a great little tool to have at your disposal.

It's also worth noting that, depending on who your carrier is, making a VoIP call over an EDGE or 3G network may violate your contract's terms of service. A little "hard" data use now and then likely won't be noticed, but if you plan on using Guava heavily with your cellular data plan, you should check and read into what's tolerated and what's not before embarking on your bold data-only adventure.

What you'll need:

Android-powered phone: Pretty much any model and nationality of the HTC Dream (T-Mobile G1), Magic (T-Mobile myTouch 3G), or Hero should work.

Google Voice and Gizmo5 accounts, hooked together: You'll have to hope for an invite if you haven't gotten your Google Voice account yet, while Gizmo is much easier to obtain. We've already shown how to connect them together in our guide to making free VoIP calls from Google Voice, under the "Hooking up Voice and Gizmo" section.

The Guava app: Head to your phone's Settings, then Applications menu, and ensure the "Unknown sources" option is checked. Then simply point your browser to that link and click the stand-alone Guava link to download the .apk installer.

Decent Wi-Fi or 3G service: At least to test out Guava and make your first test call. You can set Guava to call over EDGE connections, and even specify how many reception bars are required to try a call, but it's likely not worth the hassle.

Setting up Guava

If you've already set up Gizmo as a Google Voice phone, most of the grunt work is out of the way. Guava needs only your account names and passwords for both services to get working. That said, you'll probably want to poke around in the options (click the Menu button with the app open to get to them) to control when and how Guava makes calls over data connections. We've captured and explained Guava's options panels in a little gallery below. Click on the thumbnails for a larger view and description of the options there: