This AndroidTechie guide will have you quickly up and running making free VOIP (Wifi/3G/Edge) calls in a matter of minutes using Sipdroid and Google Voice.

Why would you want to make VOIP calls though? First, if you're overseas and want to call the states. This will work over wifi or cellular data. So as long as you have an internet connection you can call anywhere in the states for free. And they can call you for free on your google voice number. I've tried it and it works great. Second, you're out of minutes for the month. Admit it, it's happened to everyone. Now you don't have to not make a phone call or pay ridiculous overage charges. Or, you might happen to be in a place with no cell service, but you have wifi available. Oh yeah...you can also be on the phone and the web at the same time now.
If you uninstall follow the directions in this article to change the pbxes information to your secondary account.

Steps:

1. Get google voice if you don't have it. You do have to set this up from a computer as they don't allow account setup over mobile browsers. Make sure it's under the same account as the one linked to your phone (and consequently, market account). If it's not, and you don't want to make a new voice account, you can still get this to work, see post #3 below.
Install the google voice app from the market and sign in.

2. Download Sipdroid from the market and install it.

3. On initially starting the app, there will be a button at the bottom that says set up a free pbxes account to link with google voice. Press it.

4. It will automatically fill in a pbxes account name with your gmail account name. Change this if you wish, just write down what you enter.

5. You'll see right below that, it filled in the name of the gmail account linked to your phone. Again, this is important, and where I messed up, because I had a different google voice account than the account linked to my phone. For the password, make sure you enter the password for the google voice account listed.

6. Below is the Sipdroid main screen where you can make phone calls.

7. Click Menu in your phone -> Setting. In the Settings Menu: Press Call Options

8. Press Preferred Call Type

9. Set it to 'Always ask' (and choose call with Sipdroid when you get a prompt making calls).

10. Then, in the google voice app settings, set your Google Voice options to never use google voice.
In the main settings screen of Siphdroid, press the SIP Account at the top, scroll down and make sure you check WLan (Wifi) to enable calls over Wifi and check 3G and EDGE to enable cell data calls.

To dial using sipdroid, in the main phone dialer add a + to the end of the phone number like: 555-555-5555+. This will trigger sipdroid to make the call, but prevent you from getting a prompt every time you want to place a regular phone call.
Or, you can dial from the sipdroid app by entering the number on the main screen and pressing enter.

That's it! Make sure you write down or remember your pbxes account name in case you have to reinstall sipdroid.
Now, try it out. Dial as instructed in step 7 and make sure it works.
And if you're still not 100% sure this is actually working over data, do this.
- From the dialer, dial *#*#4636#*#*
- Choose Phone Information
- Scroll Down and where it says CDMA auto (prl), tap that and change it to EVDO only. This will keep your data connection alive and disable your cellular radio.
- Try Sipdroid again. Still works. Just don't forget to change your radio back to CDMA auto...
Also, with this configuration, incoming calls to google voice may reach your phone via cellular, and not Wifi / data. If you want to remove the possibility of incoming calls going over cellular, sign into google voice, and under settings, uncheck mobile. Just leave google chat checked. Now incoming calls can only forward using VOIP. Note, this setting has no influence on outgoing calls via sipdroid, they will always use a Wifi / data connection.
But if, for instance, you were overseas and using this to accept free incoming calls, you would absolutely want to make sure mobile is not checked.