Sporadic One way drop outs on CME 4.3

[exonetinf1nity]

Greetings, im having sporadic issues with a CME system where by the the called party can here the calling party but not the other way around, this only happens for about 1 in every 25 calls and originates from the CME location.

We have a Cisco 2811 running CME v4.3 and using SIP as the call method, this connects to a Cisco 2960 PoE Switch, the firewall in place is an ASA 5510 running 8.0.3(19) code and the WAN connection is a 2Mbit/s leased line terminated on an 1841 running 12.4(21)

Auto QoS is configured on the voice interface on the 2811, the 2960 switch, i have priority queues for voice on the ASA and Auto QoS configured on the 1841 serial interface.

My first assumption would be that it's a NAT timeout issue on the firewall, all the timeout values are defaulted and SIP inspection is turned on, i have configured a span port on the switch and have wireshark collecting voice statistics but could you guys suggest other logging sources i could look at to see where the problem lies or has anyone had this issue previously?

Any help would be much appreciated.

Regards

[#2]

Troubleshooting One Way Voice Issues:

http://www.cisco.com/en/US/tech/tk652/tk698/technologies_tech_note09186a008009484b.shtml

http://www.cisco.com/en/US/docs/voice_ip_comm/cucme/requirements/guide/cme43spc.htm

[exonetinf1nity]

Cheers for the links, ive narrowed down the issue which appears to only be happening a few times a day, when it does occur the jitter leaps to around 4600, this is using the call statistics feature found on the phone.

Regards