The Cisco Unity Express Auto Attendant fails to transfer inbound calls from the PSTN to the IP phone intermittently. The caller hears a failsafe that says "The system is experiencing a problem and cannot take the call right now. Try again later". The SIP/2.0 500 Internal Server Error is seen in traces.
The issue is addressed in Cisco bug ID CSCse39392.
From the trace log, there are SIP/2.0 500 Internal Server Errors after the call is transferred, after 202 accepted messages are in the traces. The 500 internal server error occurs when the Cisco CallManager Express terminates the call legs to the Cisco Unity Express, in order to finish the transfer and map the disconnect cause 16 to the SIP 500 status message.
Because the 500 and 202 messages are received in a very short time amount of time 202 accepted first, and 500 later sometimes the Cisco Unity Express processes the 500 before 202 message. This can cause the Cisco Unity Express to think that the transfer failed and play the failsafe prompt.
In order to resolve this issue, perform one of these steps:
Change the transfer-mode to blind bye-also.
Upgrade the Cisco Unity Express to version 2.3.2, which fixes the problem.