If there are free VOIP calls right now I guess it is most likely just with in free trial period from providers..I know a very cost effective service visit www.pbx101.com. Hope this could help. You can use also use skype.
Does Anyone Know Of A FREE Voip To Call Vietnam?
It's not completely free, but you can make calls to Vietnam (both mobile and landline) for just $0.08/min using CrazyCall
If you want free service of VoIP then you may want to tell your friend, relatives or family to use Skype. With Skype-to-Skype call you get instant and free VoIP for business or home calls.
Proprietary protocol such as Skype is free for PC to PC call. While calls to land line or cell phone is made with a fee.
I have searched a lot about free calls. I could not find any other than gtalk. Even with gtalk as we all know, we can't dial any number. My conclusion is to call a number, you have to pay. There is no other way out.