To note that "Lite" in this context doesn't mean any restriction, the non-lite version "Phoner" is free as well and has some additional features that are however not interesting for the portable user.
PhonerLite is freeware but not opensource. It saves its settings in the program directory. I have had it on my stick for quite a while now already, and can recommend it for it's easy configuration.
From the website:
CheersPhonerLite is a clearly arranged and easy to used application for Windows. PhonerLite enables your PC to use it for internet telephony (VoIP, Voice over IP). Pre-conditions are a full-duplex sound card, a microphone and speakers (alternatively a headset), an internet connection and a registration at a provider supporting the protocol SIP. PhonerLite supports several SIP profiles, each configurable independently. In the same way the integrated phone book and call log are easy to use.
Chris