JSFiddle - React, Tailwind, and code Playground

HTML

<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"/>

JavaScript

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;
		}
}