--- chan_sip.c.original	2008-05-03 16:14:46.000000000 +0200
+++ chan_sip.c	2008-05-03 16:18:50.000000000 +0200
@@ -2698,7 +2698,8 @@
 	if (p->outboundproxy)
 		return &p->outboundproxy->ip;
 
-	return ast_test_flag(&p->flags[0], SIP_NAT) & SIP_NAT_ROUTE ? &p->recv : &p->sa;
+	/* It MUST always use "received" to reply, even if NAT or not (#12577) */
+	return &p->recv;	
 }
 
 /*! \brief Display SIP nat mode */