It appears as though Telegram will launch call functionality in the near future, possibly in the next update. Screenshots distributed online reveal quite a bit about the things users can control with Telegram calls.
The first notable call setting is the ability to control just who can call you. Because Telegram allows you to connect with strangers via usernames - unlike WhatsApp which requires you to share a phone number - Telegram users may not want everyone to be able to ring them. Users can either allow everyone to call them, permit calls only from those on their contact lists, or simply block all incoming calls. There is also an option for blocking individual users from calling.
The other notable aspect to Telegram calls is the ability to verify that a call is encrypted. Telegram generates an image and text string which can be compared with the image and text generated on the other user’s device. If they’re the same then end-to-end encryption is guaranteed.
The calling functionality can be tested by Android users running the latest beta release of Telegram. The menu item to make calls is hidden by default but following this guide (Telegram required) will detail how to enable calls that you can experiment with on a test server.
Source: TGBeta (Telegram required)