It appears that solving this problem involves more than simply upgrading an Asterisk box. To aid in trouble shooting I am adding some details which I omitted earlier.
After the initial exchange of INVITE and 200 OK the PSTN gateway sends an ACK that the UA does not receive. The UA re-sends the 200 OK repeatedly and the gateway re-sends the ACK repeatedly. This continues for seven seconds. During this interval there is two way audio.
Weird part 1
A proper ACK with the contact in the request line finally arrives and is delivered to the UA. The UA is now happy and stops sending 200 OK.
Weird part 2
Although the UA is happy, the listener is not. At the same moment that the gateway sent a proper ACK, it also turned off its audio.
For completeness I am attaching an example of the proper ACK as seen entering and leaving the proxy.
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