MicroSIP - open source portable SIP softphone based on PJSIP stack for Windows OS. It allowing to do high quality VoIP calls (person-to-person or on regular telephones) via open SIP protocol. From cloud of SIP providers you can choose best for you, register account and use it with MicroSIP. You'll get free person-to-person calls and cheap international calls.
small footprint (>2.5MB) and RAM usage (>5MB) - written in C and C++ with minimal possible system resources usage
usability - user friendly in daily usage
functionality - voice; video H.264 and H.263+; SIMPLE messaging (RFC 3428) and presence (RFC 3903, 6665)
compatibility - strongly conform to SIP standards
voice quality - supports best voice codecs: speex@8,16,32kHz, iLBC@8kHz, GSM@8kHz, G.711@8kHz (PCMU and PCMA), G.722@16kHz, G.729@8kHz, SILK@8,12,16,24kHz, Linear PCM@8,16,44kHz
privacy - configurable encryption TLS / SRTP for control and media