server.js

app.post('/api/request-token', (req, res) => {
    let { phoneNo } = req.body;
    if (!phoneNo) return res.status(400).send({ message: "Phone no. is required" });

    phoneNo = phoneNo.replace(/\s/g, "");
    console.log("Sending code to ", phoneNo);
    
    requestCode(phoneNo, (result) => {
      if (result) {
        // success at 1st attempt
        if (result.status == '0') onOtpSuccess(res, result);

        // concurrent OTP error
        else if (result.status == '10') {
          // cancel & try again once
          cancelOtp(result.request_id, (cancelData) => {

            // cancel success
            if (cancelData && cancelData.status == '0') requestCode(phoneNo, (result2) => {

              // success at 2nd attempt
              if (result2 && result2.status == '0') await onOtpSuccess(res, result2);

              // fail again at 2nd attempt
              else onOtpError(res);
            });

            // fail at cancel
            else onOtpError(res);
          })
        }
      }

      // fail at 1st attempt
      else onOtpError(res);
    })
  } catch (e) {
    console.log(e);
    // fail server issue
    onOtpError(res);
  }
};

function onOtpSuccess(res, result) {
  // ...
  // store result.request_id on DB oe sent it to user
  res.send({ message: "Success", request_id: result.request_id });
}

function onOtpError(res) {
	//...
  res.status(500).send({ message: 'Error sending OTP. Please try again.' });
}