The 911 call is outbound on a PRI to the PSTN then to the PSAP. Due to Pay Phones, etc. the PSAP will not be held responsible for sending answer supervision back to the PSTN when they answer.

PBX's have a pseudo answer timer that accomplishes this, I do not think the CCM has such a timer. There may be a way to setup a h323 gateway for 911 calls so the answer detect is always flagged and the actual call duration is reflected on the CDR.