A demand made of a customer of a stock brokerage or futures commission merchant firm to add more money into his or her account. Margin calls are made in order to bring investorsÂ’ accounts up to a minimum level. This happens when the price of a stock, futures contract, or other security declines after it has been purchased on margin. In the futures market, a clearinghouse also may make a call asking the member to increase the amount of money on deposit.