substitute 403 to 503 of server's domestic responses
insert into scode_translations_internal (i_scode_translations_internal, in_scode, out_scode, reason) values (1, 403, 503, 'Service unavailable');

and to convert codes of remote party:

INSERT INTO scode_translations_external (in_scode, out_scode, reason,i_account_class) values(403,503,'Service unavailable',2);
to get i_account_class:

select username, i_account, i_customer, i_account_class from accounts  where i_account=(select i_account from cdrs where i_call=(select i_call from calls where call_id=''));
where call_id is the call with 403 error code.

Useful documentations:

Starting in our 5.1 Release we have added support for 3xx SIP Redirects.  For more information please see

Reason Header Field mapping RFC 3326, Q850 / Q.850, SIP codes translation

Prompts substitution of error responses