tshark -n udp and host 10.0.11.10 or host 10.0.11.21

CALL FROM 982 (10.0.11.21, Nokia E52) via Asterisk 1.6.2-svn (10.0.2.5) to 980 (10.0.11.10, Cisco 7960)
Wait some seconds before disconnect--> Phone (980) stops ringing...

	1.669701   10.0.11.21 -> 10.0.2.5     SIP/SDP Request: INVITE sip:980@10.0.2.5, with session description
	1.670159     10.0.2.5 -> 10.0.11.21   SIP Status: 401 Unauthorized
	1.716600   10.0.11.21 -> 10.0.2.5     SIP Request: ACK sip:980@10.0.2.5
	1.748020   10.0.11.21 -> 10.0.2.5     SIP/SDP Request: INVITE sip:980@10.0.2.5, with session description
	1.749178     10.0.2.5 -> 10.0.11.21   SIP Status: 100 Trying
	1.786337     10.0.2.5 -> 10.0.11.10   SIP/SDP Request: INVITE sip:980@10.0.11.10:5060;transport=udp, with session description
	1.788013     10.0.2.5 -> 10.0.11.21   SIP Status: 180 Ringing
	1.976426   10.0.11.10 -> 10.0.2.5     SIP Status: 100 Trying
	2.105515   10.0.11.10 -> 10.0.2.5     SIP Status: 180 Ringing
	2.106978     10.0.2.5 -> 10.0.11.21   SIP Status: 180 Ringing
	4.272607   10.0.11.21 -> 10.0.2.5     SIP Request: CANCEL sip:980@10.0.2.5
	4.273299     10.0.2.5 -> 10.0.11.21   SIP Status: 487 Request Terminated
	4.273331     10.0.2.5 -> 10.0.11.21   SIP Status: 200 OK
	4.275559     10.0.2.5 -> 10.0.11.10   SIP Request: CANCEL sip:980@10.0.11.10:5060;transport=udp
	4.346919   10.0.11.21 -> 10.0.2.5     SIP Request: ACK sip:980@10.0.2.5
	4.415949   10.0.11.10 -> 10.0.2.5     SIP Status: 200 OK
	4.491272   10.0.11.10 -> 10.0.2.5     SIP Status: 487 Request Cancelled
	4.491380     10.0.2.5 -> 10.0.11.10   SIP Request: ACK sip:980@10.0.11.10:5060;transport=udp

CALL FROM 982 (10.0.11.21, Nokia E52) via Asterisk 1.6.2-svn (10.0.2.5) to 980 (10.0.11.10, Cisco 7960)
Immediately disconnect --> Phone (980) keeps ringing !!!

	31.228236   10.0.11.21 -> 10.0.2.5     SIP/SDP Request: INVITE sip:980@10.0.2.5, with session description
	31.228758     10.0.2.5 -> 10.0.11.21   SIP Status: 401 Unauthorized
	31.284908   10.0.11.21 -> 10.0.2.5     SIP Request: ACK sip:980@10.0.2.5
	31.342094   10.0.11.21 -> 10.0.2.5     SIP/SDP Request: INVITE sip:980@10.0.2.5, with session description
	31.343201     10.0.2.5 -> 10.0.11.21   SIP Status: 100 Trying
	31.385927     10.0.2.5 -> 10.0.11.10   SIP/SDP Request: INVITE sip:980@10.0.11.10:5060;transport=udp, with session description
	31.388821     10.0.2.5 -> 10.0.11.21   SIP Status: 180 Ringing
	31.456435   10.0.11.21 -> 10.0.2.5     SIP Request: CANCEL sip:980@10.0.2.5
	31.457055     10.0.2.5 -> 10.0.11.21   SIP Status: 487 Request Terminated
	31.457082     10.0.2.5 -> 10.0.11.21   SIP Status: 200 OK
	31.571009   10.0.11.21 -> 10.0.2.5     SIP Request: ACK sip:980@10.0.2.5
	31.580479   10.0.11.10 -> 10.0.2.5     SIP Status: 100 Trying
	31.704213   10.0.11.10 -> 10.0.2.5     SIP Status: 180 Ringing

With patch applied (same behaviour):
 
	4.524347   10.0.11.21 -> 10.0.2.5     SIP/SDP Request: INVITE sip:980@10.0.2.5, with session description
	4.524850     10.0.2.5 -> 10.0.11.21   SIP Status: 401 Unauthorized
	4.581727   10.0.11.21 -> 10.0.2.5     SIP Request: ACK sip:980@10.0.2.5
	4.608853   10.0.11.21 -> 10.0.2.5     SIP/SDP Request: INVITE sip:980@10.0.2.5, with session description
	4.609955     10.0.2.5 -> 10.0.11.21   SIP Status: 100 Trying
	4.650964     10.0.2.5 -> 10.0.11.10   SIP/SDP Request: INVITE sip:980@10.0.11.10:5060;transport=udp, with session description
	4.651744     10.0.2.5 -> 10.0.11.21   SIP Status: 180 Ringing
	4.742704   10.0.11.21 -> 10.0.2.5     SIP Request: CANCEL sip:980@10.0.2.5
	4.743075     10.0.2.5 -> 10.0.11.21   SIP Status: 487 Request Terminated
	4.743108     10.0.2.5 -> 10.0.11.21   SIP Status: 200 OK
	4.801604   10.0.11.21 -> 10.0.2.5     SIP Request: ACK sip:980@10.0.2.5
	4.852856   10.0.11.10 -> 10.0.2.5     SIP Status: 100 Trying
	4.960296   10.0.11.10 -> 10.0.2.5     SIP Status: 180 Ringing

	29.531461   10.0.11.21 -> 10.0.2.5     SIP/SDP Request: INVITE sip:980@10.0.2.5, with session description
	29.532012     10.0.2.5 -> 10.0.11.21   SIP Status: 401 Unauthorized
	29.587906   10.0.11.21 -> 10.0.2.5     SIP Request: ACK sip:980@10.0.2.5
	29.618954   10.0.11.21 -> 10.0.2.5     SIP/SDP Request: INVITE sip:980@10.0.2.5, with session description
	29.620078     10.0.2.5 -> 10.0.11.21   SIP Status: 100 Trying
	29.660630     10.0.2.5 -> 10.0.11.10   SIP/SDP Request: INVITE sip:980@10.0.11.10:5060;transport=udp, with session description
	29.662100     10.0.2.5 -> 10.0.11.21   SIP Status: 180 Ringing
	29.728526   10.0.11.21 -> 10.0.2.5     SIP Request: CANCEL sip:980@10.0.2.5
	29.729018     10.0.2.5 -> 10.0.11.21   SIP Status: 487 Request Terminated
	29.729053     10.0.2.5 -> 10.0.11.21   SIP Status: 200 OK
	29.797827   10.0.11.21 -> 10.0.2.5     SIP Request: ACK sip:980@10.0.2.5
	29.847995   10.0.11.10 -> 10.0.2.5     SIP Status: 100 Trying
	29.966441   10.0.11.10 -> 10.0.2.5     SIP Status: 180 Ringing

When the problem occurs, the Cancel-Request from Asterisk "10.0.2.5 -> 10.0.11.10   SIP Request: CANCEL sip:980@10.0.11.10:5060;transport=udp" is missing !!!

Note:
You see different subnets because of testing from home via VPN, no NAT, Firewall or anything else between the phones and Asterisk Server that could cause the problem.
Same behaviour if all phones are in the same subnet!!!