Sipdroid

The app routes calls dialed from your phone's built-in contacts app to VoIP. In settings you can choose when to use VoIP and when to make standard phone calls, based on being logged in to Wifi, and/or based on number prefixes. Visit sipdroid.org for more info.

For optimal battery usage reserve a free VoIP PBX on pbxes.org, and manage your SIP trunks using a web browser.

Being open source, Sipdroid has often been cloned appearing under names such as Guava, aSIP, Fritz!App, ...