The users experience 10 – 20s delays when receiving inbound calls, either through Analog or VoIP trunk. Outbound and internal calls (between extensions) work as expected.
The More You Know
Please ensure that the Call Control option under the Value-added Features > API Configuration page is disabled. When this option is enabled, the UCM will wait 10 seconds for their API (if any) to control the incoming call. Even if you do not use an API but enable this option, you will still experience inbound call delays.