I've configured UAC auth to successfully register and my route[PSTN] looks like that

        # check if PSTN GW IP is defined
        if (strempty($sel(cfg_get.pstn.gw_ip))) {
                xlog("SCRIPT: PSTN routing enabled but pstn.gw_ip not defined\n");
                return;
        }
        if !ds_is_from_list()
                return;
        # route to PSTN dialed numbers starting with '+' or '00'
        #     (international format)
        # - update the condition to match your dialing rules for PSTN routing
        if(!($rU=~"^(\+|00)[1-9][0-9]{3,20}$"))
                return;
        if (strempty($sel(cfg_get.pstn.gw_port))) {
                $ru = "sip:" + $rU + "@" + $sel(cfg_get.pstn.gw_ip);
        } else {
                $ru = "sip:" + $rU + "@" + $sel(cfg_get.pstn.gw_ip) + ":"
                                        + $sel(cfg_get.pstn.gw_port);
        }

        remove_hf("To");
        insert_hf("To: <sip:$rU@sipprovider.com>\r\n", "Call-ID");
        uac_replace_from("","sip:user@sipprovider.com");
        route(RELAY);
        exit;