3CXPhone for Android is a free VoIP Softphone that works with SIP based IP PBXs such as 3CX or Asterisk and popular VoIP providers. 3CXPhone is not locked to a particular provider so you can switch easily. NOW with support for Android 2.3!

More information on http://www.3cx.com/VOIP/voip-phone.html

Phones 3CXPhone for Android has been tested with:

- HTC Wilfire S: Android 2.3.3
- HTC Wildfire : Android 2.2, build 2.22.405.1
- HTC Desire : Android 2.2
- Samsung GT-I9100 : Android 2.3.3
- Samsung GT-I9000 : Android 2.2
- Sony Ericsson Xperia U20i: Android 2.1 build 2.1.1.A.0.6
- Sony Ericsson Xperia X10i: Android 2.0, build 2.0.2.A.0.24
- Motorola XOOM: Android 3.1
- HTC EVO 4G: Android 2.3
- Asus Transformer HC 3.1

UNSUPPORTED
- Motorola Droid X
- Samsung GT-I5800 2.2.1

Known issues:

Android 1.5 & 1.6 is not supported
On Samsung GT-I5900, volume adjustment by device's volume button has no effect.

More informations under:
General: http://www.3cx.com/products/3CXPhone-for-Android.html
Setup: http://www.3cx.com/blog/3cxphone/android/configuring/
Support: http://www.3cx.com/forums/3cxphone-for-android-34/

Screenshots: