Index: channels/chan_sip.c
===================================================================
--- channels/chan_sip.c	(revision 230380)
+++ channels/chan_sip.c	(working copy)
@@ -9126,6 +9126,10 @@
 			res = AUTH_PEER_NOT_DYNAMIC;
 		} else {
 			ast_copy_flags(&p->flags[0], &peer->flags[0], SIP_NAT);
+			if ((ast_test_flag(&p->flags[0], SIP_NAT) & SIP_NAT_RFC3581) && ast_test_flag(&p->flags[1], SIP_PAGE2_RPORT_PRESENT)) {
+				ast_set_flag(&p->flags[0], SIP_NAT_ROUTE);
+			}
+
 			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)))) {
 				if (sip_cancel_destroy(p))