Sinon stub not working if tested using express app

I have a a controller function like below.
SendOTPController.js

const otpService = require('../services/otpService')


module.exports = async function(req, res) {
                 const {error, data}  = await sendOTP(req.query.phone)
                 if(error)
                     return res.send(error)
                 return res.send(data)
}

otpService.js

module.exports = async function(phone) {
                await result = fetch(`http://api.send-otp?phone=${phone}`)
                if (result !== sucess)
                    return {
                        error: "Failed to send OTP!"
                        data: null
                    }
                 return {
                        error: null
                        data: result
                    }

}

Below is my test.

const expect = require('chai').expect
const request = require('supertest')
const sinon = require('sinon')
const rewire = require('rewire')
const SendOTPController= rewire('../../src/controllers/SendOTPController')
const app = require('../../src/app')

describe('GET /api/v1/auth/otp/generate', function () {
    it('should generate OTP', async () => {
        let stub = sinon.stub().returns({
            error: null,
            data: "OTP sent"
        })
        SendOTPController.__set__('sendOTPOnPhone', stub)
        const result = await request(app)
            .get('/api/v1/auth/otp/generate?phone=8576863491')
            .set('Accept', 'application/json')
            .expect('Content-Type', /json/)
            .expect(200)
        console.log(result.body)
        expect(stub.called).to.be.true
    })
})

In above code the stub is not being called.

But if use only controller without using express app it works fine.

const expect = require('chai').expect
const request = require('supertest')
const sinon = require('sinon')
const rewire = require('rewire')
const SendOTPController= rewire('../../src/controllers/SendOTPController')
const app = require('../../src/app')

describe('GET /api/v1/auth/otp/generate', function () {
    it('should generate OTP', async () => {
        let stub = sinon.stub().returns({
            error: null,
            data: "OTP sent"
        })
        SendOTPController.__set__('sendOTPOnPhone', stub)
        const result = await SendOTPController() // not using express app, hence not passing req, res 
        console.log(result)
        expect(stub.called).to.be.true
    })
})

I went through many modules and docs.
They give a solution how I can stub a module.exports = async function(){}.
They also work, but only If they are directly imported and tested.

They don't work if I use it with express app.
Any help would be appreciated, thanks.