diff -Naur asterisk-certified-13.21-cert3/res/res_pjsip_t38.c asterisk-certified-13.21-cert3-patched/res/res_pjsip_t38.c
--- asterisk-certified-13.21-cert3/res/res_pjsip_t38.c	2018-09-20 21:04:02.000000000 +0200
+++ asterisk-certified-13.21-cert3-patched/res/res_pjsip_t38.c	2019-07-16 11:54:00.694532261 +0200
@@ -289,6 +289,35 @@
 	return 0;
 }
 
+/*! \brief Callback for when a response is received for a T.38 fallback re-invite */
+static int t38_fallback_response_cb(struct ast_sip_session *session, pjsip_rx_data *rdata)
+{
+	struct pjsip_status_line status = rdata->msg_info.msg->line.status;
+	struct ast_format *fmt;
+	
+	if (status.code / 100 == 2) {
+		ast_debug(2, "channel %s accepted t38 audio fallback, setting native formats...\n",
+			ast_channel_name(session->channel));
+		fmt = ast_format_cap_get_best_by_type(ast_channel_nativeformats(session->channel), AST_MEDIA_TYPE_AUDIO);
+		if (!fmt) {
+				/* No format? That's weird. */
+				ast_log(LOG_WARNING, "Received response to T.38 fallback re-invite on '%s' but format is not available!!!\n",
+				session->channel ? ast_channel_name(session->channel) : "unknown channel");
+				return 0;
+		}
+		ast_channel_set_writeformat(session->channel, fmt);
+		ast_channel_set_rawwriteformat(session->channel, fmt);
+		ast_channel_set_readformat(session->channel, fmt);
+		ast_channel_set_rawreadformat(session->channel, fmt);
+		ao2_ref(fmt, -1);
+	} else {
+		ast_debug(2, "channel %s refused audio fallback... don't know what to do...",
+			ast_channel_name(session->channel));
+	}
+	return 0;
+	
+}
+
 /*! \brief Callback for when a response is received for a T.38 re-invite */
 static int t38_reinvite_response_cb(struct ast_sip_session *session, pjsip_rx_data *rdata)
 {
@@ -307,6 +336,16 @@
 		return 0;
 	}
 
+	if (status.code / 100 != 2) {
+		ast_debug(2, "t38 refused on channel %s, falling back to G711",
+			ast_channel_name(session->channel));
+		ast_format_cap_remove_by_type(session->req_caps, AST_MEDIA_TYPE_AUDIO);
+		ast_format_cap_update_by_allow_disallow(session->req_caps, "!all,alaw,ulaw", 1);
+		ast_sip_session_refresh(session, NULL, NULL, t38_fallback_response_cb, AST_SIP_SESSION_REFRESH_METHOD_INVITE, 1);
+		ast_debug(2, "t38 fallback invite sent on channel %s, falling back to G711, waiting for response...\n",
+			ast_channel_name(session->channel));
+	}
+
 	t38_change_state(session, session_media, state, (status.code == 200) ? T38_ENABLED : T38_REJECTED);
 
 	ao2_cleanup(session_media);