If I use shadows call on a unit at a battlefield and hide temporal breach, can I react to the temporary trigger with the hidden temporal breach to save the unit?

Yes — as long as Temporal Breach was hidden on a previous turn and it's hidden at the same battlefield as the unit.

The Temporary trigger from Shadow's Call goes on the chain at the start of your Beginning Phase, and that chain creates the reaction window (FAQ #6571, #6741). By then the hidden Temporal Breach has [Reaction] — hidden cards gain it "beginning on the next turn" (Rule 811.1.b), so the same-turn-hide restriction doesn't block this.

When Temporal Breach resolves, the unit is banished and replayed to the same battlefield as a new game object — the Temporary granted by Shadow's Call doesn't carry over, and the pending "kill this" trigger references the old object, which no longer exists, so the kill instruction is ignored (Rule 359.3.e.12). The unit survives.

Two catches: the hidden Temporal Breach can only target a unit at the battlefield where it's hidden (Rule 811.1.d.2), and if you hid it this turn, you can't play it yet — you'd have to hide it a turn ahead of the kill trigger.