diff -r -u asterisk-1.4.18/channels/chan_sip.c asterisk-1.4.18-mod/channels/chan_sip.c
--- asterisk-1.4.18/channels/chan_sip.c	2008-01-31 14:52:49.000000000 -0500
+++ asterisk-1.4.18-mod/channels/chan_sip.c	2008-03-06 13:30:06.000000000 -0500
@@ -15375,6 +15375,9 @@
 		append_history(p, "Rx", "%s / %s / %s", req.data, get_header(&req, "CSeq"), req.rlPart2);
 
 	if (!lockretry) {
+		p = find_call(&req, &sin, req.method);  /* returns p locked */
+		if (p == NULL) return 1; /* We couldn't lock since it was removed. */
+
 		if (p->owner)
 			ast_log(LOG_ERROR, "We could NOT get the channel lock for %s! \n", S_OR(p->owner->name, "- no channel name ??? - "));
 		ast_log(LOG_ERROR, "SIP transaction failed: %s \n", p->callid);
@@ -15382,6 +15385,7 @@
 			transmit_response(p, "503 Server error", &req);	/* We must respond according to RFC 3261 sec 12.2 */
 		/* XXX We could add retry-after to make sure they come back */
 		append_history(p, "LockFail", "Owner lock failed, transaction failed.");
+		ast_mutex_unlock(&p->lock);
 		return 1;
 	}
 	nounlock = 0;