Plenty of VoIP apps are available in the Android Market, but I had to do my homework to find the right one for me. At first I thought I could use the T-Mobile Wi-Fi Calling app, which comes preinstalled on the phone, but I discovered that calls made through that app still eat up cellular minutes. Next I turned to Google Voice, which I already use for making calls from my PC for free. All I had to do was download and install the app from the Android Market, and it would integrate seamlessly into the Dialer app. Unfortunately, Google Voice calls consume mobile minutes, too, so I wasn't able to use it for this experiment.
After dabbling with Fring and looking into setting up Sipdroid, I finally settled on using Skype for my VoIP needs, since I already had an account and I use it fairly regularly on my PC and iPad. Skype's rates depend on how much you plan to use your phone, as well as on where you're located. Since I'm in the United States, and since I don't make a lot of lengthy calls, I opted to stick with the standard rate, which costs 2.3 cents per minute--about one-quarter of what T-Mobile's plan would charge me for voice. Skype offers various unlimited-usage monthly plans as well, ranging from $3 a month for unlimited calls within the U.S. and Canada to $14 a month for unlimited calls worldwide. And if you want to receive calls via Skype, you'll need to shell out an extra $6 a month for an Online Number. It isn't free, but $9 for unlimited calls out of a static number is still pretty cheap.
Skype-Only for a Week
Overall, the arrangement worked pretty well. Oddly enough, my 4G reception was the worst in my office and my apartment, but since Wi-Fi covers both areas just fine, I didn't have to worry about missing a call in the two places I spend 90% of my time. I live in the San Francisco Bay Area, which T-Mobile's 4G network seems to cover reasonably well, considering that I didn't have much of a problem placing or receiving calls via Skype.
The call quality was great. Generally it didn't sound as good as Skype sounds on a PC with a solid broadband connection, but it still sounded much better than a normal cellular voice call. I did have a few issues with echoing, dropped calls, and occasional spurts of lag--all of which were rather frustrating--and I'd routinely drop calls going through 3G/4G dead spots (passing through Treasure Island on the Bay Bridge, for example) that the T-Mobile voice network handles capably.