Phone screen virtual pianos are always fun to try, if a little impractical, yet they usually disappoint in terms of responsiveness and polyphony. My Piano Phone (available for Windows Phone 8.1 but also as a full Windows 10 UWP as far as I can tell) gets everything right - sounding authentic, with perfect key response, plus a myriad of alternative voices and preloaded example tunes. Oh, and did I mention that it's a labour of love for the developer and is totally free?

From the Store description:

Enjoy real Piano sound from My Piano Phone:

• Acoustic Piano, Electric Piano, Acoustic Guitar, Electric Guitar, Trumpet and Violin
• Lot of songs to play and learn
• Full Keyboards of Piano
• Chords
• Multi Touch
• One, two or mirror keyboards at a time
• Drumkit
• And more...

The use cases, other than general fun, are in composing (though there's no record facility), i.e. getting ideas 'down' (on manuscript paper, after hearing how phrases sound?), and learning the basics of the piano or keyboard. 

Here's My Piano Phone in action, here on a Lumia 950 XL, and note that you'll need to swipe away the Windows 10 navigation controls if you want to see the whole keyboard and all top-of-screen controls:

The main interface, you can obviously select which part of the piano you want to 'see' and, in fact, how many keys, i.e. you can trade width for octaves, see the Settings below...

You can also select your voice/instrument, all of which are beautifully sampled, you can transpose up or down in semitones, and you can even change the keyboard to play chords rather than notes, if you're accompanying someone else, for example.

You also get over 400 MIDI-style example tunes to enjoy and learn, right up to 21 Pilots, so pretty up to date!!

You can download My Piano Phone in the Store here, highly recommended.