Today, you would have to write a "relay" application that is installed directly on the 3CX server. This relay would have to connect to and subscribe to events exposed by the 3CX Call Control API. Then your relay would have to call your webhook. There is no built-in support for doing this. However, it has been announced that 3CX V20 will eventually provide a new API for subscribing to these real-time events. No information has been provided about the technology used to subscribe to these events. It could be webhooks or SignalR or something else. No details have been provided by 3CX yet. I saw a discussion here in the forums that suggested that the APIs will not be available in the initial release of V20, but may show up in SP1 or SP2, but that's just a rumor.
I will, however, mention that we offer a "Relay" today that allows you to subscribe to real-time events, as do other 3rd-party developers. We implement event subscriptions using SignalR, but there is at least one other partner that has implemented a webhooks approach.
