Does anybody have an example about how to use credentials to permit some kinds of calls to some users on SER?

                if (uri=~"^sip:0800.*@.*") {
                        if(!is_user_in("credentials", "0800")) {
                                sl_send_reply("403", "No 0800");
                                log(1,"No 0800");
                                break;
                        };
                        consume_credentials();
                        prefix ("5511");
                        route(8);
                        break;
                };