Index: channels/chan_sip.c
===================================================================
--- channels/chan_sip.c	(revision 186569)
+++ channels/chan_sip.c	(working copy)
@@ -14433,13 +14433,40 @@
 	}
 	
 	if (!ast_test_flag(req, SIP_PKT_IGNORE) && p->pendinginvite) {
-		/* We already have a pending invite. Sorry. You are on hold. */
-		p->glareinvite = seqno;
-		transmit_response_reliable(p, "491 Request Pending", req);
-		if (option_debug)
-			ast_log(LOG_DEBUG, "Got INVITE on call where we already have pending INVITE, deferring that - %s\n", p->callid);
-		/* Don't destroy dialog here */
-		return 0;
+		if (!ast_test_flag(&p->flags[0], SIP_OUTGOING) && ast_test_flag(&p->flags[1], SIP_PAGE2_DIALOG_ESTABLISHED)) {
+			/* We have received a reINVITE on an incoming call to which we have sent a 200 OK but not yet received
+			 * an ACK. According to RFC 5407, Section 3.1.4, the proper way to handle this race condition is to accept
+			 * the reINVITE since we have established a dialog.
+			 *
+			 * So what needs to be done to insure that everything goes smoothly? First, we need to clear
+			 * the p->pendinginvite since it no longer is pending. Next, we need to cancel the retransmission
+			 * of our 200 OK in response to the initial INVITE. When we later receive the ACK for the initial INVITE,
+			 * we should just ignore it since the iseqno for this sip_pvt has increased beyond the CSeq in the initial
+			 * INVITE transaction.
+			 *
+			 * Basically, we're rolling the ACK and reINVITE handling into one here.
+			 */
+
+			/* Note that this will both clear the pendinginvite flag and cancel the 
+			 * retransmission of the 200 OK
+			 */
+			__sip_ack(p, p->lastinvite, FLAG_RESPONSE, 0);
+
+			/* At this point, we've cleared the pendinginvite number off the sip_pvt, and stopped
+			 * the 200 OK retransmissions. The big item now is to make sure that when we do
+			 * receive an ACK for the initial INVITE request that we do just ignore it.
+			 *
+			 * The rest of the processing for the reINVITE should just work
+			 */
+		} else {
+			/* We already have a pending invite. Sorry. You are on hold. */
+			p->glareinvite = seqno;
+			transmit_response_reliable(p, "491 Request Pending", req);
+			if (option_debug)
+				ast_log(LOG_DEBUG, "Got INVITE on call where we already have pending INVITE, deferring that - %s\n", p->callid);
+			/* Don't destroy dialog here */
+			return 0;
+		}
 	}
 
 	p_replaces = get_header(req, "Replaces");