i hvae a unit at battlefield A and B, can i move a unit from battlefield B to A with call to battle even if my opponent doesnt have an unit

Yes, you can. Call to Battle only requires you to move a unit you control to a battlefield you control — since you have a unit on A, it's a valid destination, and spells (unlike the Standard Move) have no battlefield-to-battlefield restriction. The opponent having no unit just means the second half of the spell does nothing for them; it's not a condition for the spell to be legal.

Moving a unit to a battlefield you already control doesn't apply Contested or stage a Showdown (Rule 450; FAQ #4407). If the opponent did have a unit and moved it to A, that would Contested the field and start a Showdown — but with no unit available, nothing happens.