[Serusers] 503 service unavailable

[zhaomin]

I use asterisk as a pstn and voicemail,but I get this error from asterisk 's CLI blow:
Verbosity is at least 3
    -- Executing AbsoluteTimeout("SIP/61.183.23.216-08a3b558", "15") in new stack
    -- Set Absolute Timeout to 15
    -- Executing Congestion("SIP/61.183.23.216-08a3b558", "") in new stack
  == Spawn extension (from-sip-external, 2000, 2) exited non-zero on 'SIP/61.183.23.216-08a3b558'
    -- Executing AbsoluteTimeout("SIP/61.183.23.216-08a3b558", "15") in new stack
    -- Set Absolute Timeout to 15
    -- Executing Congestion("SIP/61.183.23.216-08a3b558", "") in new stack
  == Spawn extension (from-sip-external, h, 2) exited non-zero on 'SIP/61.183.23.216-08a3b558'
What's wrong in it? ser or asterisk? I am not sure .anyones would help me?
Thanks 
Zhaommin
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.sip-router.org/pipermail/sr-users/attachments/20050910/a8a480d3/attachment.htm>