NodeJS - Getting result from asynchronous function: cannot read property 'then' of undefined

I have this function on NodeJS:

const easysoap = require('easysoap');

function request(action, inputField) {

  const clientParams = {
    host: 'https://example.com/demo/service',
    path: '/service.asmx',
    wsdl: '/service.asmx?WSDL',
    headers: [{
      name: 'SOAPAction',
      value: `service/GetNumber`
    }]
  };
  const clientOptions = {
    secure: false
  };
  const soapClient = easysoap.createClient(clientParams, clientOptions);

  soapClient.call({
    method: "GetNumber",
    attributes: {
      xmlns: 'service'
    },
    params: {
      Credentials: {
        UserName: 'Me',
        Password: 'mypass'
      }
    }
  })
  .then((callResponse) => {
    return callResponse.data.GetNumberResponse.number;
  })
  .catch((err) => {
    console.log('Got an error making SOAP call: ', err);
  });
}

export default { request };

The request is working well if invoked via curl as a web service. However, transformed as a function I always get error TypeError: Cannot read property 'then' of undefined when running the test below:

describe('# GetInvoiceDetailsbyInvoiceNumber', () => {
    it('should return some valid number', (done) => {
      soap.request()
        .then((output) => {
          console.log(output);
          expect(output).to.deep.equal(expectedOutput);
          done();
        })
        .catch(done);
    });
});

What is wrong?

via Arturo