We have an iMac and a MacBook Air, both running OSX 10.9.5 Mavericks. We do not have "smart" phones. I looked at Viber and WhatsApp but it seems both require a smart phone as well in order to run on a desktop or laptop computer. This rules them out for us, unless they have now come out with a standalone desktop version for Mac. ie, just download and install it on either or both Macs and then start using it, just as we originally did with Skype, without any connection to a smart phone or other device.
Our need for a replacement is because Skype has just cut us off because we have not "upgraded" to some later version. The one we have has worked fine - nothing needed to be upgraded. Now the idiots refuse to let us use it.
Have you considered 'FaceTime' ? An Apple product
AFAIK and seems to be popular.