HTML - input TELEFONO con los codigos de cada pais

<select id="pais">

<option value="">SELECCIONE UN PAIS</option>

<option value="+54">ARGENTINA</option>

<option value="+56">CHILE</option>

<option value="+52">MEXICO</option>

</select>

<br>

<input type="text" id="telf" disabled="disabled"/>

<script>

var pais = document.getElementById('pais');

var telf = document.getElementById('telf');

pais.onchange = function(e) {

	telf.value = this.value;

	if((this.value).trim() != '') {

		telf.disabled = false;

	} else {

		telf.disabled = true

	}

}

telf.onkeyup = function(e) {

	var nums_v = this.value.match(/\d+/g);

	if (nums_v != null) {

		this.value = '+'+((nums_v).toString().replace(/\,/, ''));

	} else {

		this.value = pais.value;

	}

}

</script>