Mask in html field does not work

Good afternoon,

I have a page in my application to register clients, nothing out of the ordinary.

In this page, I have some fields (cif, rg, phone, telephone_2, etc.) that when typing are formatted with javascript with the following function:

* Transforming the phone: 1199999999 at (11) 9999-9999

// Masks Javascript

function mascara(o,f){
    v_obj=o
    v_fun=f
    setTimeout("execmascara()",1)
}

function execmascara(){
    v_obj.value=v_fun(v_obj.value)
}

function id( el ){
    return document.getElementById( el );
}

// aqui começa as mascaras 

function mtel(v){ //MASCARA PARA TELEFONE

    v=v.replace(/\D/g,"");             //Remove tudo o que não é dígito
    v=v.replace(/^(\d{2})(\d)/g,"($1) $2"); //Coloca parênteses em volta dos dois primeiros dígitos
    v=v.replace(/(\d)(\d{4})$/,"$1-$2");    //Coloca hífen entre o quarto e o quinto dígitos
    return v;
}

window.onload = function atribuiMascaras(){ // FUNCAO QUE É ACIONADO AO CARREGAR A PAGINA ( WINDOW.ONLOAD )

    id('txtCel').onkeyup = function(){ //ATRIBUI O CAMPO COM ID txtCel A MASCARA DE TELEFONE
        mascara( this, mtel );
    }

It turns out that, I can access this page and already have partially the data of this contact that is becoming a client.

If I already have this partial data, when I access the page, I get the ID of the contact in the Url and I make a call in a controller to get the data of that client.

$(document).ready(function selectDadosContato(){        

var id_contato = <?=$dados[0];?>;
var tipo = '<?=$dados[1];?>';

jQuery.ajax
    ({  

        url: "/admin2/controllers/contatos/Contatos.php?id="+id_contato+"&tipo="+tipo, //URL de destino
        dataType: "json", //Tipo de Retorno
        success:
        function(data) {    

                var d = data.data_nascimento;
                var d = d.split('-');
                var data_nasc = d[2] + '/' + d[1] + '/' + d[0];

                document.getElementById('txtNome').value = data.nome;
                document.getElementById('txtCel').value = data.telefone;                    

             } //function(data);    
        });
});

Here is my problem, when I make the request and get the contact data and assign the value received in the html fields, I would need the masks of the fields to be activated, but the fields are filled the way they come from the bank .

Ha ha, grateful