I have had a Rebtel International account for many years. It costs $10 U.S./mo. with unlimited minutes, and can access almost any landline OR cellphone worldwide (including 800 #s etc). Call quality is uniformly excellent. Although we mostly use WhatsApp, I will always keep Rebtel. It has been a lifesaver for those unfortunate times I've had to spend hours or days dealing with U.S. banks, government agencies, or whatever.
In case anyone is wondering, all I need do is dial a CABA access number (because I live in the Buenos Aires area). Then a voice prompt requests that I enter the country code, area code and the number I wish to call, followed by the # key. Moments later the call is ringing.
P.S. They may offer a calling card option, which you can pay as you go, but I'm not sure as I've never used it.