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There is either one-way or no-way audio between IP phones in CallManager version 3.3
This issue is documented in Cisco bug ID CSCeb19725. Intermittent one-way or no-way audio is experienced for the first 10 seconds between Cisco IP Phones (no gateway involved).
Cisco CallManager traces show the calls being connected but no voice stream is being passed.
A review of a sniffer trace between phones shows that the streams do not get established until the phone issues an Address Resolution Protocol (ARP) request roughly 10 seconds after the skinny message comes in to cut through audio.
To resolve this issue, perform these steps:
Issue the netstat -es command under Cisco CallManager and see if there are a large number of Transmission Control Protocol (TCP) retransmissions. If there are a large number of TCP retransmissions, check these options:
The switch should be set to 100 MB full duplex.
If the Cisco CallManager is set to auto, change it to 100 MB full duplex.
Upgrade loads to the latest release included in cmterm-7940-7960.P003-5-0-3.exe.
Note: This problem was recently exposed because of the operating system upgrade in Cisco CallManager version 3.3 and also that the transmission starts after an ARP request. This issue is fixed in the load provided with Cisco CallManager version 3.3(3)SR.
In addition, this is a signed load. Once loaded, the system cannot go back to an unsigned load. It is also fixed in Cisco CallManager version 3.3(3)SR2.