Jquery Phone number restriction

jquery_phone_number_restriction.js

// Phone number restriction for input_10_4 field

// Start with + character
  
$('#input_10_4').val('+');
$('#input_10_4').attr('data-initial', '+');

// Cannot delete the + character

$("#input_10_4").on("keyup", function() {
	var value = $(this).val();
	$(this).val($(this).data("initial") + value.substring(1));
});

// Enter only numeric characters

$("#input_10_4").on("keypress", function(event) {
	if (event.charCode < 48 || event.charCode > 57) {return false;} //keyCode is not working on Firefox
});