It is possible to call directly from Odoo if some WebRTC softphone application (e.g. Odoo VoIP) is installed.
But our modules does not work like that. We use Asterisk API to control calls and get call events. This approach makes our apps nearly universal as they work with nearly all Asterisk distros and are VoIP phone agnostic so you can use any phones. But there is one requirement - you must have an Asterisk server.