Presently you can use something like Groove IP or Talkatone that utilize Google voice to make free phone calls to the US from another US number. That will change on May 15th when Google will take away the VOIP services that have been using Google Voice.
There are Voip services that you can use as a Google Voice replacement. I have been looking into them just in case Google decides to stick to us android users. I don't see that as likely though as iOS devices are able to now. But, I wanted to cover my bases just in case
Be aware that most of the Voip services will only let you receive inbound calls for free, Not all services give you a number. Out bound calls do cost but are cheap. Several of the Voips charge like 1 cent a minute. You pay for minutes in advance. From what I can gather those minutes don't expire.
I found on just yesterday, Upptalk, that provides a number and give you free calling inbound and out bound to US phone number from the US phone number they offer.