A new alpha build of Telegram for desktop has been released with the inclusion of support for calls. Calling functionality already launched a few months ago on Android and iOS, with Windows Phone users still waiting. It is looking as though the desktop app which is available on Windows, Mac, and Linux will receive the calls update at the same time as the macOS update.
Once launched on desktop, it'll be proper to consider Telegram a proper Skype rival in a way that WhatsApp doesn't quite qualify, mainly because it doesn't have comprehensive desktop support like its two rivals. It's interesting to note where Telegram and Skype started from; Telegram originated on mobile, while Skype started out on desktop; this may be part of the reason why Telegram was recently shown to use much less data than Skype in a recent study.
It should be noted that, on the call side of things, Telegram has not yet reached feature parity with Skype. For instance, it doesn't yet support group calls or video calls nor are these features present in any development builds available for the public. Given the rapidity of Telegram's development, however, it's not unrealistic to expect those features sooner rather than later.