I was wandering through the Diamondcard site and re-read something I had seen before:
Diamondcard contributes, promotes and supports the following open source projects: Ekiga, Twinkle, Asterisk, FreeSWITCH, OpenSIPS, WikiPBX, Podwiki, FreeRADIUS, Linux, FreeBSD, Apache, Perl, Python, GNU arch. Our entire platform is built using these great tools.
Diamondcard funds Twinkle and Ekiga projects. The more calls you make with Ekiga and Twinkle softphones using Diamondcard, the more money donated to these projects. 10% of every recharge or online shop purchase is donated to these open source developers.
Diamondcard handles this kind of thing well. They put up detailed instructions how to use FOSS (like Ekiga and Twinkle) with their service, and donate back to the kinds of projects that they use or who send customers their way.
My current setup is that Voice rings both my TMO line (Android phone) and Ekiga on the desktop, set up to listen for Gizmo calls. Works fine.