The best way to do it on blur is:
function formatPhone(obj) {
if (obj.value != "")
{
var numbers = obj.value.replace(/\D/g, ''),
char = {0:'(',3:') ',6:' - '};
obj.value = '';
upto = numbers.length;
if(numbers.length < 10)
{
upto = numbers.length;
}
else
{
upto = 10;
}
for (var i = 0; i < upto; i++) {
obj.value += (char[i]||'') + numbers[i];
}
}
}