MicroSIP is a free portable SIP softphone for Windows based on PJSIP stack. It allowing to do high quality VoIP calls (P2P 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 p2p calls and cheap international calls.
Main in MicroSIP:
small RAM usage (< 10 MB) - written in C and C++ with minimal possible system resources usage usability - user friendly in daily usage functionality - voice, messaging, presence compatibility - strongly conform to SIP standards voice quality - supports best voice codecs portability - stores setting in ini file