Allow dynamic method calls in JS, but no dynamic property access

I’d like to allow dynamic method calls on a JavaScript object, i.e. even if the object does not have the method defined, it should still be invokable (e.g. with a default dummy).

const obj = {a: 5};
obj.dynamicMethodCallWithAnyNameYouWant(); // should work
obj.a; // should work, value is 5
obj.b; // should yield undefined

Now with a JS Proxy, we can do something like this to get in the right direction:

const obj = new Proxy({a: 5}, {
  get(target, prop, receiver) {
    // If the property does not exist, define it as a new function
    if (!(prop in target)) {
      target[prop] = function() {
        console.log(`Method ${prop} has been dynamically created and invoked!`);
      };
    }
    return Reflect.get(target, prop, receiver);
  }
});

obj.dynamicMethodCallWithAnyNameYouWant();
console.log(obj.a); // prints 5
const res = obj.thisShouldYieldUndefinedInsteadOfCreatingAFunction
res(); // this prints "Method thisShouldYieldUndefined ..."

The problem is that we cannot detect whether the property was accessed or called since

[…] JavaScript does not distinguish between attributes and methods. It’s all just properties that are accessed, and their value can be called if it’s a function. ~ this answer

Is there another way to achieve the desired behavior, e.g. with an alternative approach to proxies?