For having a more clear code, and having the code ready for new features in the future, I would change this to `addToExistingContact(phoneNumber, successHandler, errorHandler)`.
With this approach, inside the `try-catch`, I would add `activity.onsuccess` & `activity.onerror` and, if the handler exist, I will execute it! With this model we could add more logic if needed in the future.

In https://github.com/mozilla-b2g/gaia/pull/8696/files#L2R52 you should only define, in this case, `successHandler`, for hiding the `options-menu` if needed.