I am using Anveo as I believe it is the cheapest although not exactly free. Close to it. I did not find their set up to be intuitive, but after reading the help files I figured it out and am using Zoiper on my computer as a phone. I also ported m number to Anveo and have that forwarded to google voice , as I prefer the google voice answering program. I have the cheapest Anveo account with the ported number which is not unlimited, but I would have to make a huge number of calls to go over. Every thing ends up on my cell phone so I use that for many of my calls. I think it might have more to do with what device you are using than the provider. I use a Obihai 202 Again, I want to say that not being a VOIP expert, it took me some time to figure out how to get every thing to work together.
Some of my friends use phone power but do not use Zoiper.