Index: channels/chan_sip.c
===================================================================
--- channels/chan_sip.c	(revision 181422)
+++ channels/chan_sip.c	(working copy)
@@ -4006,7 +4006,7 @@
 		break;
 	case AST_CONTROL_CONGESTION:
 		if (ast->_state != AST_STATE_UP) {
-			transmit_response_reliable(p, "503 Service Unavailable", &p->initreq);
+			transmit_response_reliable(p, "500 Server Internal Error", &p->initreq);
 			p->invitestate = INV_COMPLETED;
 			sip_alreadygone(p);
 			ast_softhangup_nolock(ast, AST_SOFTHANGUP_DEV);