Index: channels/chan_sip.c
===================================================================
--- channels/chan_sip.c	(revision 41987)
+++ channels/chan_sip.c	(working copy)
@@ -3221,18 +3221,14 @@
 				/* stop retransmitting an INVITE that has not received a response */
 				__sip_pretend_ack(p);
 
-				/* if we can't send right now, mark it pending */
-				if (!ast_test_flag(&p->flags[0], SIP_CAN_BYE)) {
-					ast_set_flag(&p->flags[0], SIP_PENDINGBYE);
-				} 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. */
-					ast_clear_flag(&locflags, SIP_NEEDDESTROY);
+				/* 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. */
+				ast_clear_flag(&locflags, SIP_NEEDDESTROY);
 
-					sip_scheddestroy(p, SIP_TRANS_TIMEOUT);
-				}
+				sip_scheddestroy(p, SIP_TRANS_TIMEOUT);
+
 				if ( p->initid != -1 ) {
 					/* channel still up - reverse dec of inUse counter
 					   only if the channel is not auto-congested */
@@ -11235,10 +11231,18 @@
 	    (resp != 183))
 		resp = 183;
 
+	/* Section 12.1 of RFC3261 says a dialog isn't established on a provisional, 1xx, response
+		unless there is a tag in the To field, aka an "early dialog". Additionally, section 15
+		states that a BYE _CANNOT_ be sent in response to an INVITE unless the INVITE has been
+		ACKed, not pending.  If there is no To tag or the INVITE has not been ACKed, 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))
 			sip_cancel_destroy(p);
+		if (!ast_strlen_zero(p->theirtag) && !p->pendinginvite)
+			ast_set_flag(&p->flags[0], SIP_CAN_BYE);
 		check_pendings(p);
 		break;
 	case 180:	/* 180 Ringing */
@@ -11257,7 +11261,8 @@
 				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 */
@@ -11271,7 +11276,8 @@
 				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 */