Index: channels/chan_sip.c
===================================================================
--- channels/chan_sip.c	(revision 213882)
+++ channels/chan_sip.c	(working copy)
@@ -3552,7 +3552,7 @@
 			return "404 Not Found";
 		case AST_CAUSE_CONGESTION:		/* 34 */
 		case AST_CAUSE_SWITCH_CONGESTION:	/* 42 */
-			return "503 Service Unavailable";
+			return "500 Service Unavailable";
 		case AST_CAUSE_NO_USER_RESPONSE:	/* 18 */
 			return "408 Request Timeout";
 		case AST_CAUSE_NO_ANSWER:		/* 19 */
@@ -4059,7 +4059,7 @@
 		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 Service Unavailable", &p->initreq);
 			p->invitestate = INV_COMPLETED;
 			sip_alreadygone(p);
 			ast_softhangup_nolock(ast, AST_SOFTHANGUP_DEV);
@@ -13987,7 +13987,7 @@
 		}
 
 		/*
-		From the RFC...
+		From RFC 3515
 		A minimal, but complete, implementation can respond with a single
    		NOTIFY containing either the body:
       			SIP/2.0 100 Trying
@@ -14039,6 +14039,7 @@
 			/* Do we get the header in the packet in this case? */
 			success = FALSE;
 			break;
+		case 500:   /* Internal error: The new call failed */
 		case 503:	/* Service Unavailable: The new call failed */
 				/* Cancel transfer, continue the call */
 			success = FALSE;
@@ -15020,9 +15021,9 @@
 				if (ast_pickup_call(c)) {
 					ast_log(LOG_NOTICE, "Nothing to pick up for %s\n", p->callid);
 					if (ast_test_flag(req, SIP_PKT_IGNORE))
-						transmit_response(p, "503 Unavailable", req);	/* OEJ - Right answer? */
+						transmit_response(p, "500 Unavailable", req);	/* OEJ - Right answer? */
 					else
-						transmit_response_reliable(p, "503 Unavailable", req);
+						transmit_response_reliable(p, "500 Unavailable", req);
 					sip_alreadygone(p);
 					/* Unlock locks so ast_hangup can do its magic */
 					ast_mutex_unlock(&p->lock);
@@ -16245,7 +16246,7 @@
 			if (option_debug)
 				ast_log(LOG_DEBUG, "Ignoring too old SIP packet packet %d (expecting >= %d)\n", seqno, p->icseq);
 			if (req->method != SIP_ACK)
-				transmit_response(p, "503 Server error", req);	/* We must respond according to RFC 3261 sec 12.2 */
+				transmit_response(p, "500 Server error", req);	/* We must respond according to RFC 3261 sec 12.2 */
 			return -1;
 		}
 	} else if (p->icseq &&