Index: channels/chan_sip.c
===================================================================
--- channels/chan_sip.c	(revision 44332)
+++ channels/chan_sip.c	(working copy)
@@ -2475,15 +2475,16 @@
 
 				/* are we allowed to send CANCEL yet? if not, mark
 				   it pending */
-				if (!ast_test_flag(p, SIP_CAN_BYE)) {
+/*				if (!ast_test_flag(p, SIP_CAN_BYE)) {
 					ast_set_flag(p, SIP_PENDINGBYE);
+*/
 					/* Do we need a timer here if we don't hear from them at all? */
-				} else {
+/*				} else { */
 					/* Send a new request: CANCEL */
 					transmit_request_with_auth(p, SIP_CANCEL, p->ocseq, 1, 0);
 					/* 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. */
-				}
+/*				} */
 				if ( p->initid != -1 ) {
 					/* channel still up - reverse dec of inUse counter
 					   only if the channel is not auto-congested */
@@ -9616,6 +9617,12 @@
 	    (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 (!ignore)
@@ -9637,7 +9644,8 @@
 				ast_queue_control(p->owner, AST_CONTROL_PROGRESS);
 			}
 		}
-		ast_set_flag(p, SIP_CAN_BYE);
+		if (!ast_strlen_zero(p->theirtag) && !p->pendinginvite)
+			ast_set_flag(p, SIP_CAN_BYE);
 		check_pendings(p);
 		break;
 	case 183:	/* Session progress */
@@ -9651,7 +9659,8 @@
 				ast_queue_control(p->owner, AST_CONTROL_PROGRESS);
 			}
 		}
-		ast_set_flag(p, SIP_CAN_BYE);
+		if (!ast_strlen_zero(p->theirtag) && !p->pendinginvite)
+			ast_set_flag(p, SIP_CAN_BYE);
 		check_pendings(p);
 		break;
 	case 200:	/* 200 OK on invite - someone's answering our call */