Trigger change event when key pressed in knockout

I am trying to trigger change event when key is pressed. I subscribed to the observable as shown below but it only detect change after focus is changed.

Edit: for an option binding:

<input class="form-control" type="text" data-bind="inputmask: { value: homePhone, mask: selectedHomePhoneFormat }"/>

in js:

self.homePhone = ko.observable('');

Subscribe:

 self.homePhone.subscribe(function (newvalue) {
    console.log("changed", newvalue);
});

Above code only detect change after I change focus. I want it to detect change every time when key pressed.