By default, call waiting is enabled on your configuration. In order to disable this feature at system level please go to the below setting:
Configuration > Extensions > Click on desired Ext > Scroll down to "Call waiting"
Always dial the extension, relay on phone Call Waiting settings: This is the default setting and will allow you to see if there is another incoming call whilst you are on the phone.
Check if the extension is INUSE and report as BUSY: Selecting this option disables the call waiting feature and the incoming call will receive a busy tone if you are already on a call.