this is the ngrep -record  when i tried to make a incoming call from PSTN




U 81.21.38.13:57812 -> 81.21.33.35:5060
INVITE sip:22030980@81.21.33.35:5060 SIP/2.0.
Via: SIP/2.0/UDP  81.21.38.13:5060.
From: <sip:22498045@81.21.38.13>;tag=78415468-CC3.
To: <sip:22030980@81.21.33.35>.
Date: Fri, 15 Sep 2006 09:15:20 GMT.
Call-ID: 8BB4320C-43D111DB-9F91880F-F0154510@81.21.38.13.
Supported: timer,100rel.
Min-SE:  1800.
Cisco-Guid: 3763255713-1164120539-2201223171-3126568985.
User-Agent: Cisco-SIPGateway/IOS-12.x.
Allow: INVITE, OPTIONS, BYE, CANCEL, ACK, PRACK, COMET, REFER, SUBSCRIBE, NOTIFY, INFO.
CSeq: 101 INVITE.
Max-Forwards: 6.
Remote-Party-ID: <sip:22498045@81.21.38.13>;party=calling;screen=yes;privacy=off.
Timestamp: 1158311720.
Contact: <sip:22498045@81.21.38.13:5060>.
Expires: 180.
Allow-Events: telephone-event.
Content-Type: application/sdp.
Content-Length: 274.
.
v=0.
o=CiscoSystemsSIP-GW-UserAgent 9485 5123 IN IP4 81.21.38.13.
s=SIP Call.
c=IN IP4 81.21.38.13.
t=0 0.
m=audio 17124 RTP/AVP 18 0 8 100.
a=rtpmap:18 G729/8000.
a=fmtp:18 annexb=no.
a=rtpmap:0 PCMU/8000.
a=rtpmap:8 PCMA/8000.
a=rtpmap:100 X-NSE/8000.
a=fmtp:100 192-194.

#
U 81.21.33.35:5060 -> 81.21.38.13:5060
SIP/2.0 407 Proxy Authentication Required.
Via: SIP/2.0/UDP  81.21.38.13:5060.
From: <sip:22498045@81.21.38.13>;tag=78415468-CC3.
To: <sip:22030980@81.21.33.35>;tag=74961b5b71b6ddce908b9155b956083f.58b5.
Call-ID: 8BB4320C-43D111DB-9F91880F-F0154510@81.21.38.13.
CSeq: 101 INVITE.
Proxy-Authenticate: Digest realm="81.21.33.35", nonce="450a6fac7fc0d4000fd13a85f7f6c55323ed6a5c".
Content-Length: 0.
Warning: 392 81.21.33.35:5060 "Noisy feedback tells:  pid=24505 req_src_ip=81.21.38.13 req_src_port=57812 in_uri=sip:22030980@81.21.33.35:5060 out_uri=sip:22030980@81.21.33.35:5060 via_cnt==1".
.

#
U 81.21.38.13:57812 -> 81.21.33.35:5060
ACK sip:22030980@81.21.33.35:5060 SIP/2.0.
Via: SIP/2.0/UDP  81.21.38.13:5060.
From: <sip:22498045@81.21.38.13>;tag=78415468-CC3.
To: <sip:22030980@81.21.33.35>;tag=74961b5b71b6ddce908b9155b956083f.58b5.
Date: Fri, 15 Sep 2006 09:15:20 GMT.
Call-ID: 8BB4320C-43D111DB-9F91880F-F0154510@81.21.38.13.
Max-Forwards: 6.
Content-Length: 0.
CSeq: 101 ACK.
.

#
U 81.21.38.13:51826 -> 81.21.33.35:5060
INVITE sip:22030980@81.21.33.35:5060 SIP/2.0.
Via: SIP/2.0/UDP  81.21.38.13:5060.
From: <sip:22498045@81.21.38.13>;tag=78415970-1FEC.
To: <sip:22030980@81.21.33.35>.
Date: Fri, 15 Sep 2006 09:15:22 GMT.
Call-ID: 8C78BCF5-43D111DB-9F95880F-F0154510@81.21.38.13.
Supported: timer,100rel.
Min-SE:  1800.
Cisco-Guid: 3776566429-1164120539-2202206211-3126568985.
User-Agent: Cisco-SIPGateway/IOS-12.x.
Allow: INVITE, OPTIONS, BYE, CANCEL, ACK, PRACK, COMET, REFER, SUBSCRIBE, NOTIFY, INFO.
CSeq: 101 INVITE.
Max-Forwards: 6.
Remote-Party-ID: <sip:22498045@81.21.38.13>;party=calling;screen=yes;privacy=off.
Timestamp: 1158311722.
Contact: <sip:22498045@81.21.38.13:5060>.
Expires: 180.
Allow-Events: telephone-event.
Content-Type: application/sdp.
Content-Length: 274.
.



And these messages are repeating until the requested time out;-)

Whats wrong here :-

               Hope this information can help me

                      Thanks,
Ravi.