1) Alarm Clock can play only to network connected devices (DLNA or AirPlay), or USB sound card connected to NAS.
2) Alarm Clock uses QNAP MusicStation API for playing sound. So if you see your device in devices list in MusicStation, and can play to that device in MusicStation, so you can use it in Alarm Clock.
3) You can’t select your phone as playing device directly in Music Station.
But, you could install to your phone special app that will act as network player, and then it may be used by MusicStation / AlarmClock.
For example, for Android (tested by me) – https://play.google.com/store/apps/details?id=com.waxrain.airplayer
I think there are similar apps for iphone
(see https://itunes.apple.com/ru/app/airplayer/id369713694?mt=8 for example)