If you're using a newer version of Android, you can now enjoy a built-in SIP/VoIP (Voice over Internet Protocol) functionality. This is available for devices that use the Google Android operating system.

The built-in SIP/VoIP functionality allows you to make and receive calls over the Internet, even if your phone is not connected to the Internet. It works like a regular telephone, with the exception that calls are made over the Internet, instead of a traditional phone line.

The app allows you to switch between two modes. The first mode is the default mode, where the app will make calls using your default settings. The second mode is the SIP mode, where the app will make calls over the Internet, but with the option to make calls via your normal phone settings as well.

The widget has two buttons. The left one gives you a shortcut to the SIP settings. The right one allows you to select how calls should be routed.