A margin call is a demand to add collateral or reduce exposure because a position is approaching its maintenance requirement. In traditional brokerage it is literally a call or notice with a deadline. In crypto derivatives the concept mostly survives as a warning level in the interface, because the risk engine does not wait for anyone.
01How crypto handles it
Exchanges send email and push notifications when the margin ratio crosses warning thresholds, often at 1.5x and 1.25x maintenance. Those notifications are a courtesy. The binding event is the ratio touching 1.0, at which point the position is liquidated instantly. There is no deadline to meet and no human to call.
The practical margin call is therefore the margin ratio bar in the trading interface. Traders who manage risk set their own action level, adding margin or cutting size well before the bar turns red.
02Why the warning stage matters
The distance between your warning level and liquidation is your reaction window. On high leverage that window can be a single candle. On moderate leverage it is hours or days, which is the difference between a position you can defend and one that dies while you sleep.