--- asterisk-1.4.23/channels/chan_sip.c	2009-01-13 19:34:14.000000000 +0100
+++ asterisk-1.4.23-patched/channels/chan_sip.c	2009-03-06 13:35:30.000000000 +0100
@@ -9119,6 +9135,9 @@
 		if (!strcmp(sip_pvt_ptr->callid, callid)) {
 			int match = 1;
 
+			if (option_debug > 3)
+				ast_log(LOG_DEBUG, "Found call with callid %s (ourtag=%s, theirtag=%s)\n", callid, sip_pvt_ptr->tag, sip_pvt_ptr->theirtag);
+
 			/* Go ahead and lock it (and its owner) before returning */
 			ast_mutex_lock(&sip_pvt_ptr->lock);
 
@@ -9129,16 +9148,21 @@
 			if (pedanticsipchecking) {
 				const char *pvt_fromtag, *pvt_totag;
 
-				if (ast_test_flag(&sip_pvt_ptr->flags[1], SIP_PAGE2_OUTGOING_CALL)) {
-					/* Outgoing call tags : from is "our", to is "their" */
-					pvt_fromtag = sip_pvt_ptr->tag ;
-					pvt_totag = sip_pvt_ptr->theirtag ;
-				} else {
-					/* Incoming call tags : from is "their", to is "our" */
-					pvt_fromtag = sip_pvt_ptr->theirtag ;
-					pvt_totag = sip_pvt_ptr->tag ;
-				}
-				if (ast_strlen_zero(fromtag) || strcmp(fromtag, pvt_fromtag) || (!ast_strlen_zero(totag) && strcmp(totag, pvt_totag)))
+				/* RFC 3891
+				  > 3.  User Agent Server Behavior: Receiving a Replaces Header
+				  > The Replaces header contains information used to match an existing
+				  > SIP dialog (call-id, to-tag, and from-tag).  Upon receiving an INVITE
+				  > with a Replaces header, the User Agent (UA) attempts to match this
+				  > information with a confirmed or early dialog.  The User Agent Server
+				  > (UAS) matches the to-tag and from-tag parameters as if they were tags
+				  > present in an incoming request.  In other words, the to-tag parameter
+				  > is compared to the local tag, and the from-tag parameter is compared
+				  > to the remote tag.
+
+				  Thus, the totag is always compared to the local tag, regardless if
+				  this our call is an incoming or outgoing call.
+				*/
+				if (ast_strlen_zero(fromtag) || strcmp(fromtag, sip_pvt_ptr->theirtag) || (!ast_strlen_zero(totag) && strcmp(totag, sip_pvt_ptr->tag)))
 					match = 0;
 			}