For Mac and iOS users, an alternative to Skype would be FaceTime, which allows Mac and iOS users to video call one another through their phone numbers or Apple IDs. Unfortunately for Mac, FaceTime is still limited to video calls, which iOS users have the option of placing audio-only FaceTime calls. Well the good news is that Mac users can look forward to the audio-only option in a future OS X Mavericks update, thanks to the latest beta OS X 10.9.2 that is currently being seeded to developers.
While Apple did not make any formal announcement regarding this feature, it was discovered by developers that FaceTime audio had been integrated into the likes of the Messages and FaceTime app, hinting that the feature could be officially introduced in a future OS X Mavericks update. Similar to Skype, users will be able to seamlessly switch between audio and video thanks to a new button, as well as mute the chat.
According to developers who have tried the feature, apparently FaceTime audio on iOS devices offers a better-sounding call, and that it takes a while to stabilize. Presumably these are some of the kinks Apple is working on ironing out which hopefully will be resolved once the feature goes live for the public.