I just tried it (gtalk2voip) last weekend and it worked fine. (Normally I use skype, but the person I was trying to call had computer broke due to thunderstorm... and only way to reach her was calling to cellphone. I just happened to have unused credit with Gtalk2voip, and used it as backup... )

EDIT: Didn't reply how to sign up... It has been so long time that I do not remember. But if I'm not completely mistaken you just add a new "user" (service@gtalk2voip.com) to your gtalk contacts, start "chat" with "him", and there you go... check gtalk2voip.com web site for further instructions.