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 ...