Trying to get property of non-object, can't access to phoneNumber properties

Route::get('/', function (){

$contacts = Contact::all();
//dd($contacts);

return view('welcome')->with('contacts',$contacts);

});

From the route i pass the $cotacts to the view

@foreach($contacts as  $contact)

    <li> 

        <small>by </small>

    </li>

@endforeach

but when i what to access the number of phoneNumber, this is my class:

public function phoneNumber()
    {
         return $this->hasOne(PhoneNumber::class);
    }

i get error Trying to get property of non-object ???

also when i put like this:

<li> 

            <small>by </small>

        </li>

i get: Colton Kilback IV by {"id":20,"contact_id":1,"number":"+1-936-288-3493","created_at":"2018-04-01 20:14:16","updated_at":"2018-04-01 20:14:16"}

Shaun Streich DVM by

Dr. Ruthe Thiel III by

Laverne Mertz by {"id":11,"contact_id":4,"number":"(769) 844-4643 x59321","created_at":"2018-04-01 20:14:16","updated_at":"2018-04-01 20:14:16"}

and so on some name don't have number.

via Chebli Mohamed