Trouble to get contacts from the device

I have a development of an app using the Ionic4 Framework in which I must import the native contacts book of the device using the plugin @ionic-native/contacts . What happens is that the values corresponding to the displayName and phoneNumbers parameters are not properly returned; in fact, they return null values. For demonstration purposes I put an alert() that receives the parameters to show my problem:

I also share the code fragment where I make the call of the plugin and use it:

import { Contacts, Contact, ContactField, ContactName  } from '@ionic-native/contacts/ngx';
...

export class ImportarContactosPage {

   listaContactos: any;

  constructor(public contacts: Contacts) {
    this.listarContactos();
   }

...

 listarContactos(){
    this.contacts.find(["*"])
    .then(res => {
// Alert
      alert(JSON.stringify(res));
//
      let datosMostar:any[]=[];
      res.map((item) =>{
        if(item.displayName != null && item.photos != null && item.phoneNumbers != null){
          datosMostar.push({displayName:item.displayName,photos:[{value:this.avatar}],phoneNumbers:item.phoneNumbers})
        }        
      })
      console.log(datosMostar);
      this.listaContactos = datosMostar;
    },error => {
      console.log({error:error});
    })
  }
...
}

my ionic-info :

Ionic CLI                     : 5.0.0 (C:\Users\Carlos\AppData\Roaming\npm\node_modules\ionic)
   Ionic Framework               : @ionic/angular 4.0.0
   @angular-devkit/build-angular : 0.11.4
   @angular-devkit/schematics    : 7.1.2
   @angular/cli                  : 7.1.0
   @ionic/angular-toolkit        : 1.2.0

Cordova:

   Cordova CLI       : 9.0.0 (cordova-lib@9.0.1)
   Cordova Platforms : android 8.0.0

Utility:

   cordova-res :  0.3.0
   native-run  :  0.2.5

System:

   Android SDK Tools : 26.1.1 (C:\Users\Carlos\AppData\Local\Android\Sdk)
   NodeJS            : v10.16.0 (C:\Program Files\nodejs\node.exe)
   npm               : 6.9.0
   OS                : Windows 7

I have been “googling” and reviewing forums for several days and I have tried in many ways to import contacts, but I have not been able to correct the problem.

Any fellow idea, have I done something wrong? Any suggestion is welcome. Thank you.

So now you are finish your app.