[Serusers] 503 service unavailable

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