The information on this post is way out of date by now. Please do post a link to an updated guide in the comments if you have one.
This guide will help you get a free US number (DID), and receive incoming calls on your Android device *for free* without using minutes on your voice plan, purely over Wifi (or data if you have a good data plan). This guide is part of my larger guide on how to make free calls to USA and Canada using Google Voice. 1) Register for a free Sipgate One account 2) Once you are signed up and your account is active, make note of your Sipgate SIP ID and SIP password. 3) Configure either your built-in SIP client (Android 2.3 and up), or download one of CSipSimple, Sipdroid, or Linphone from the market, and install it on your Android phone. General instructions on setting up Sipgate with a SIP client can be found here. a) For Android 2.3 and up, you may choose to use the built-in SIP functionality of your device. Go to Settings->Call Settings->Accounts. Select "Receive incoming calls" (this is essential for callback to work), and then hit "Add account". Enter your Sipgate ID and password in the appropriate fields, and sipgate.com under "Server". b) Once you install and start Sipdroid, hit Menu->Settings->SIP Account (Line 1), and enter your Sipgate ID in the "Authorization Username" field, and password in the "Password" field, and sipgate.com under "Server or Proxy". If you so choose, you can check "Use 3G" (see NOTE below about data usage). If you experience weird audio issues, you can also select Settings->Advanced Options->Use STUN Server, and enter stun.sipgate.net in "STUN Server name", or use the pre-supplied STUN server. c) Recent versions of CSipSimple have a Sipgate wizard which can be followed to set up your Sipgate account easily. When when you first start it up: - Select whether you want to be "Always available" or "Available on WiFi", depending on whether you have a generous mobile data plan or not. Also, if using your data plan, check "I'm allowed to use mobile" below. - On the next screen, click "Add account", then choose the Sipgate wizard, and enter your Sipgate ID and password in the appropriate fields, and sipgate.com under "Server". - If you experience weird networking issues, one way audio or other issues, you can try enabling STUN, Menu->Settings->Network->Enable STUN. You can elect to use callwithus STUN server (stun.sipgate.net) or the CSipSimple provided server (Thanks for your comment, Ross). - Finally, you if you want to use your mobile data plan (see NOTE below about data usage), check "Use 3G" in the settings. d) After installing Linphone, when it is first run, it will do an echo cancellation calibration. After that, you will be in the SIP Account screen. Enter your Sipgate ID and password in the appropriate fields, and sipgate.com under "Domain". You can check "Start at boot time" below, and enter Sipgate's STUN server (stun.sipgate.net) under "Stun server" if you have strange audio problems. Linphone will make use of your mobile data plan by default, so either make sure you have a generous enough mobile data plan (see NOTE below about data usage), or make sure you don't make calls when you are outside WIFI coverage. (2 - 3 minutes) NOTE: If you have a mobile data plan, you can tell Sipdroid, CSipSimple, and Linphone to connect even over your wireless provider's data plan (3G), but *beware of potentially very high data charges*, since voip can be very data intensive. In my personal tests, a high quality codec consumes about 1MB per minute. 4) You can either use this number as your real phone number, add it to Google Voice, and make GV forward to it, even use it to activate your GV account. Finally, you can use this number to MAKE FREE CALLS TO USA AND CANADA using my other guide.