I had a look at the manual and it is a sip extension. What have you set on the server for register timer.
This is also a note on the manual
Configure the Dial settings:
Quick Press for Calling Center: Optional. This option is applied to a residential call center. When a guest presses the Dial button, the call will be received on iVMS-4200 client.
Note: In our scenario, do NOT check this option, or the incoming calls cannot reach the DS-KH6320 Indoor Station.