var customOptions = {
  onKeyPress: function(val, e, field, options) {

    if (val.replace(/\D/g, '').length===2)
    {
        val = val.replace('8','');    
        field.val(val);
     }
     field.mask("+7 (999) 999-99-99", options);
    },
    placeholder: "+7 (___) ___-__-__" 
};

$("input[name='phone']").mask("+7 (999) 999-99-99", customOptions);