Fair so what do you do when a client is long or short a futures contract and margin is exhausted? When and how do you issue the call and how much time do you give clients?
In his example, that is not what he asked. He asked what happens from early assignment from a vertical call spread. That means you end up being long call and short stock, which is a hedge position with a limit to losses, but in a reg-T account, an increase in margin, that cause a call.
Your example is not the same.