This has been reported on 3M self checkouts in both the US and UK. Sip working ok for days (weeks even) then suddenly the server refuses to accept connections. Restarting sip does not always rectify (immediately) the problem. Suggests the socket gets stuffed (technical term covering a number of possible error conditions). A rare occurence and not easily repeatable. May occur intermittently when the symptom is very poor response time (sip messages are not being responded to, are timing out then server responds on second or third retry)
Closing as resolved. This may have been caused by a configuration issue. If spare servers is set to zero the server will not ensure that a server is available to answer incoming connections. When a unit attempts to reconnect all existing servers may be sitting on old connections they still think are valid (waiting for the next request that never comes), as no spare has started up the connection is refused