Index: channels/chan_sip.c
===================================================================
--- channels/chan_sip.c	(revision 65852)
+++ channels/chan_sip.c	(working copy)
@@ -786,6 +786,7 @@
 #define SIP_PAGE2_RFC2833_COMPENSATE    (1 << 25)	/*!< 25: ???? */
 #define SIP_PAGE2_BUGGY_MWI		(1 << 26)	/*!< 26: Buggy CISCO MWI fix */
 #define SIP_PAGE2_OUTGOING_CALL         (1 << 27)       /*!< 27: Is this an outgoing call? */
+#define SIP_PAGE2_REGISTERTRYING	(1 << 28)	/*!< Whether or not to send 100 Trying responses to REGISTERs */
 
 #define SIP_PAGE2_FLAGS_TO_COPY \
 	(SIP_PAGE2_ALLOWSUBSCRIBE | SIP_PAGE2_ALLOWOVERLAP | SIP_PAGE2_VIDEOSUPPORT | \
@@ -8371,7 +8372,8 @@
 			res = AUTH_PEER_NOT_DYNAMIC;
 		} else {
 			ast_copy_flags(&p->flags[0], &peer->flags[0], SIP_NAT);
-			transmit_response(p, "100 Trying", req);
+			if (ast_test_flag(&peer->flags[1], SIP_PAGE2_REGISTERTRYING))
+				transmit_response(p, "100 Trying", req);
 			if (!(res = check_auth(p, req, peer->name, peer->secret, peer->md5secret, SIP_REGISTER, uri, XMIT_UNRELIABLE, ast_test_flag(req, SIP_PKT_IGNORE)))) {
 				sip_cancel_destroy(p);
 
@@ -10142,7 +10144,8 @@
 		print_codec_to_cli(fd, &peer->prefs);
 		ast_cli(fd, ")\n");
 
-		ast_cli(fd, "  Auto-Framing:  %s \n", peer->autoframing ? "Yes" : "No");
+		ast_cli(fd, "  Auto-Framing : %s \n", peer->autoframing ? "Yes" : "No");
+		ast_cli(fd, "  100 on REG   : %s\n", ast_test_flag(&peer->flags[1], SIP_PAGE2_REGISTERTRYING) ? "Yes" : "No");
 		ast_cli(fd, "  Status       : ");
 		peer_status(peer, status, sizeof(status));
 		ast_cli(fd, "%s\n",status);
@@ -16198,6 +16201,8 @@
 			ast_parse_allow_disallow(&peer->prefs, &peer->capability, v->value, 1);
 		} else if (!strcasecmp(v->name, "disallow")) {
 			ast_parse_allow_disallow(&peer->prefs, &peer->capability, v->value, 0);
+		} else if (!strcasecmp(v->name, "registertrying")) {
+			ast_set2_flag(&peer->flags[1], ast_true(v->value), SIP_PAGE2_REGISTERTRYING);
 		} else if (!strcasecmp(v->name, "autoframing")) {
 			peer->autoframing = ast_true(v->value);
 		} else if (!strcasecmp(v->name, "rtptimeout")) {
Index: configs/sip.conf.sample
===================================================================
--- configs/sip.conf.sample	(revision 65852)
+++ configs/sip.conf.sample	(working copy)
@@ -494,6 +494,7 @@
 ;                             sendrpid
 ;                             outboundproxy
 ;                             rfc2833compensate
+;                             registertrying
 
 ;[sip_proxy]
 ; For incoming calls only. Example: FWD (Free World Dialup)