With regard to AABB (penny stock) margin calls...please see the following :
Summary 1 Non-marginable securities cannot be purchased on margin and need to be funded with the investor’s cash. 2 The securities exist to protect investors by reducing the risk that comes with purchasing securities with leverage. 3 Non-marginable securities include penny stocks, OTC stocks, and recent IPOs, as the securities tend to be riskier due to higher price volatility and lower liquidity. 4 Using margin can amplify returns but significantly increases losses when the price of the underlying security does not move in the expected direction.