This is normal behavior. If CUCM sees the remote destination call leg transition to a connected state after the Answer Too Soon timer but before the Answer Too Late timer it believes the user has answered the call. There is no way to tell whether it was the user or the carrier's voicemail that answered unfortunately.
CUCM 9.0 added a new option on the Remote Destination Policy called SNR Voicemail Policy. There are two settings: Timer Control and User Control. Timer control is the existing behavior; if the call is answered within the timer window the call is connected through. Timer Control is new and requires the user to press a DTMF digit to accept the call after answering it. Since carrier voicemail can't press DTMF digits the call would not be considered answered if the user pressed ignore.
In my case, when remote destination (Mobile Phone) refeuses a call, IP phone keeps ringing.
Not all cell phones take the same action when you ignore/refuse a call. Some simply treat that as a request to be silent while others actually forward the call to voicemail at that point. Those cell phones that continue to alert silently would allow the IP Phone to continue ringing until the CFNA timer expires.
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