Index: channels/chan_sip.c
===================================================================
--- channels/chan_sip.c	(revision 44365)
+++ channels/chan_sip.c	(working copy)
@@ -3247,17 +3247,17 @@
 				__sip_pretend_ack(p);
 
 				/* if we can't send right now, mark it pending */
-				if (!ast_test_flag(&p->flags[0], SIP_CAN_BYE)) {
+/*				if (!ast_test_flag(&p->flags[0], SIP_CAN_BYE)) {
 					ast_set_flag(&p->flags[0], SIP_PENDINGBYE);
-					/* Do we need a timer here if we don't hear from them at all? */
-				} else {
+*/					/* Do we need a timer here if we don't hear from them at all? */
+/*				} else { */
 					/* Send a new request: CANCEL */
 					transmit_request_with_auth(p, SIP_CANCEL, p->ocseq, XMIT_RELIABLE, FALSE);
 					/* Actually don't destroy us yet, wait for the 487 on our original 
 					   INVITE, but do set an autodestruct just in case we never get it. */
 					needdestroy = 0;
 					sip_scheddestroy(p, DEFAULT_TRANS_TIMEOUT);
-				}
+/*				} */
 				if ( p->initid != -1 ) {
 					/* channel still up - reverse dec of inUse counter
 					   only if the channel is not auto-congested */
@@ -11329,6 +11329,12 @@
 	    (resp != 183))
 		resp = 183;
 
+	/* According to 12.1 of RFC3261, a dialog isn't established on a provisional, 1xx, response
+		unless there is a tag in the To field, ie. an "early dialog".
+		Furthermore, section 13.2.2.1 allows for the creation of "early dialogs" if there is a tag
+		in the To field. If there is no To tag, there is no dialog, and therefore
+		a BYE _cannot_ be sent, a CANCEL _must_ be sent instead.
+	*/
 	switch (resp) {
 	case 100:	/* Trying */
 		if (!ast_test_flag(req, SIP_PKT_IGNORE))
@@ -11351,7 +11357,8 @@
 				ast_queue_control(p->owner, AST_CONTROL_PROGRESS);
 			}
 		}
-		ast_set_flag(&p->flags[0], SIP_CAN_BYE);
+		if (!ast_strlen_zero(p->theirtag) && !p->pendinginvite)
+			ast_set_flag(&p->flags[0], SIP_CAN_BYE);
 		check_pendings(p);
 		break;
 	case 183:	/* Session progress */
@@ -11365,7 +11372,8 @@
 				ast_queue_control(p->owner, AST_CONTROL_PROGRESS);
 			}
 		}
-		ast_set_flag(&p->flags[0], SIP_CAN_BYE);
+		if (!ast_strlen_zero(p->theirtag) && !p->pendinginvite)
+			ast_set_flag(&p->flags[0], SIP_CAN_BYE);
 		check_pendings(p);
 		break;
 	case 200:	/* 200 OK on invite - someone's answering our call */