[SR-Users] 503 Server error response for REGISTER request (Asterisk v1.6.0.5)

Hello, I had posted this question earlier on asterisk-users, but didn't receive any reply, so I'm posting on sr-users in the hope that someone can provide guidance on how to debug this problem. Our setup involves a Sip softphone registering with a fresh install of Asterisk v1.6.0.5 through an outbound proxy (Kamailio v3.1.1). We observe that about 80% of the time, immediately after startup of the softphone, when the softphone sends a REGISTER message, Asterisk replies with a "503 Server error". However, subsequent REGISTER messages are replied to appropriately (with a 401 Unauthorized). There are very few devices (under 5) registered with Asterisk, so this is not an overload issue. Please find attached a SIP trace for device registration. - What might cause this behavior in Asterisk ? - Are there any config file parameters that require to be changed ? - Would it help to see any other specific logs, if so which ones ? Thanks and Regards, Vikram.