Which one is the most natural: "keep your phone at/on hand," or "keep your phone...

Which one is the most natural: keep your phone at hand, keep your phone on hand or keep your phone with you? For example:

1. I might call you tonight, so please keep your phone at hand.

2. I might call you tonight, so please keep your phone on hand.

3. I might call you tonight, so please keep your phone with you.

What I want to tell a person to keep their phone where they can see and hear it when I call.

Both are possible, as is "keep your phone to hand." But you are most likely to say "keep your phone handy".