Why second javascript function is not being called?

You are returning from the onkeypress handler immediately after calling the first function:

onkeypress="return isNumberKey(event);calculate();"

In javascript everything that follows a return statement is ignored. So what you want here is to have a function that first detects if the argument is a number and the calculate:

function handlePress(evt) {
    var result = isNumberKey(evt);
    calculate();
    return result;
}

and then:

onkeypress="return handlePress(event);"

You could have written this logic in the HTML directly without introducing a third function but IMHO mixing HTML and javascript like this is bad. I would even attach this onkeypress event unobtrusively instead of using the onkeypress attribute on the input:

window.onload = function() {
    var c8 = document.getElementById('c8');
    if (c8 != null) {
        c8.onkeypress = function(evt) {
            var result = isNumberKey(evt);
            calculate();
            return result;
        };
    }
};

and in the html:

<input type="text" name="cbh" id="c8" value="0" />

Now we have a clear separation between javascript and markup. Another advantage of this is that your markup size is reduced and since static resources such as javascript files are cached you will be limiting the bandwidth usage and speeding up your site.

onkeypress="var r = isNumberKey(event);calculate();return r;"

the second function is called after the event returns. it will never be reached by the interpreter. changing to

var isNumber = isNumberKey(evt);calculate(); return isNumber;

fixes that particular problem, but you'll have cross browser issues and obtrusive code to deal with

return will end whatever function you are in (including the anonymous one generated by the onclick attribute).

You probably want something like:

var value = isNumberKey(event);
calculate();
return value;

or

onclick="var value = isNumberKey(event);calculate();return value;"

Since you have a return statement, the second statement won't be reached.

You'd be better off having one handler function and then dispatching out to more functions, e.g

onkeypress=myonkeypress

function myonkeypress() {
  if (isNumberKey()) {
    calculate();
  }
}

I would suggest placing the alert() call inside the function you are trying to debug.

Since it is only for debugging, it makes for less code to clean up once you are done debugging: you only have to remove your alert statements and you don't have to worry about anything else (i.e. removing extra function call from your onkeypress event.

So you can get rid of your calculate() function and simply update your isNumberKey function like so:

function isNumberKey(evt)
{
    var charCode = (evt.which) ? evt.which : event.keyCode
    if (charCode > 31 && (charCode < 48 || charCode > 57))
    {
         alert("isNumberKey FALSE");
         return false;
    }

    alert("isNumberKey TRUE");
    return true;
}