Hotline is a feature where lifting up the handset (offhook) automatically initiates a call to a determined number, e.g. in lobbies or emergency calls etc. The outgoing call is triggered using the phone's Action URL on offhook. (Action URLs are HTTP GET requests that are issued when a specific event occurs on the phone.)
To configure the Action URLs, go to the web user interface of the phone and click on Action URL Settings.
In the field On offhook, insert the following URL:
You need to replace the IP address by the actual IP address of the phone (alternately you can use local host or 127.0.0.1), and Extension by the number of the hotline, i.e. the number the phone should dial automatically on the event offhook.