Can you tell us where this packet capture was taken? I suspect it is from the application server host..?
A typical error scenario looks like the below:
Packet 10137
Time: 33.198154000
POST /CGI/Execute HTTP/1.1
Content-Length: 406
Packet 11679
Time: 35.960246000
HTTP/1.1 200 OK
<CiscoIPPhoneError Number="6" />
As there is >2 seconds between request and response, I would also suspect that the phone is timing out trying to establish a connection with the page it is supposed to retrieve (or possibly the Authorization URL.) As the retrieve URL seems to be the same host as the requestor - and per Nick's observation the app host network connection may be experiencing congestion - this may indicate that the server is overwhelmed and is unable to serve the page quickly enough, causing the phone to time out.
If you can manage to obtain a pcap from a phone (e.g. by plugging in a PC to the phone's extra switch port), this would give a clearer picture of what's happening at the phone during this 2 seconds. My expectation would be we would see the phone attempting to establish TCP connections and timing out. If not (for example if the phone appears to do nothing for 2 seconds and just returns the error) we may need to look at the onboard phone logs to see what's happening...
