I have 1 unit at base my opponent has 1 unit at battlefield A, can i play call to battle and move my unit to battlefield A?

No. Call to Battle only lets you move a unit to a battlefield you control, and battlefield A is controlled by your opponent — they have a unit there, so you don't control it (Rule 190.4.a).

The card's first instruction ("Move a unit you control to a battlefield you control") is a destination restriction, not just a suggestion — battlefield A fails it, so the spell can't be played that way. You'd need to first establish control of battlefield A (e.g., by moving/playing a unit there yourself) before Call to Battle could target it.