lightweight SIP softphone
Download from PortableApps.com
MicroSIP Portable can run from a cloud folder, external drive, or local folder without installing into Windows. It's even better with the PortableApps.com Platform for easy installs and automatic updates.
MicroSIP allows you to make high quality VoIP calls via open SIP protocol. You can select the best SIP provider for you. You'll get free person-to-person calls and cheap international calls. MicroSIP has an easy to use interface, support for all major voice codecs, support for SIP functionality (voice, messaging, presence, video h264 and h263+) and privacy (configurable encryption TLS / SRTP for control and media).