Index: channels/chan_sip.c
===================================================================
--- channels/chan_sip.c	(revision 380212)
+++ channels/chan_sip.c	(working copy)
@@ -14755,7 +14755,12 @@
 
 	p->pendinginvite = p->ocseq;	/* Remember that we have a pending NOTIFY in order not to confuse the NOTIFY subsystem */
 
-	return send_request(p, &req, XMIT_RELIABLE, p->ocseq);
+	/* RFC6665 4.2.2.  Sending State Information to Subscribers
+	 * If the NOTIFY request fails due to expiration of SIP Timer F (transaction timeout),
+	 * the notifier SHOULD remove the subscription.
+	 */
+
+	return send_request(p, &req, XMIT_CRITICAL, p->ocseq);
 }
 
 /*! \brief Notify user of messages waiting in voicemail (RFC3842)