Are we talking about calls to international landlines in particular? Since if its mostly to other smartphones or something similar there are many options that can work (depending on what you and the other party might have installed on their device as far as a third party service/app goes).
Being one of the few that dated my now wife who was then from another country... I used a variety of things:
LocalPhone (I put a $6/mo local phone number in her country for her to call - still have it for her family in Peru).
Nowadays we use primarily Facetime, Hangout Video Calls, and LocalPhone to communicate with family back in Peru. When internet is out (for them or us) we'll use Google Voice and LocalPhone to call landlines/cellphones.
Note: I tried EVERYTHING back in the day. Yes, even used Rebtel for awhile but found GV and Skype to be the best.
I used to get phone calls from someone who claimed to use Rebtel, the quality was horrendous. Where are you calling from? States?
I just use Skype occasionally, the quality depends on the internet connection. I actually just got a phone call from someone overseas today, she used Skype, she said my voice was crisp, I could barely hear her.
I have unlimited calls to Canada and the US and 15 or so European countries with my provider here in Switzerland - Sunrise. Otherwise I use Nonoh or skype. All both to landlines or mobiles. When I travel I keep in touch also with Viber or Facetime depending on who I'm talking to.
Kinda funny. I'm an American who lives abroad, and Google Hangouts has been my savior.
A couple years ago, Google Voice didn't have a native app, and you had to use apps like Talkatone which had their own servers to route the calls through, but now that voice is integrated into Hangouts, it's all I use to call the states.
Keep in mind, the only reason I can use it is because I signed up for a GV number when I still lived in America. I don't think most people can use Hangouts the same way that I do.