Call to Battle

Q1: I play <<Call to Battle>> and target the battlefield I control and my unit that is already at the battlefield. Is this a legal play?

A1: No.

Q2: Can I use <<Flash>> on a unit that is in base?

A2: Yes.

Rules for Context - 355.4., 355.4.a.

The difference between these is that Flash doesn’t choose a destination and thus isn’t required to choose a valid destination per 355.4. and 355.4.a., for the same reason that Azir, Ascendant can choose a unit at the same location as him. The move action will be ignored, but the target is legal.