If you want something completely free and open-source, MicroSIP is a fantastic choice. It is lightweight, has a small memory footprint, and is updated regularly by the community. It doesn't have the flashy skin of Eyebeam, but it is rock-solid for audio calls.
: A widely used alternative that supports voice, video, and instant messaging across Windows, Mac, Linux, and mobile devices. Eyebeam Download With License Key