as a finance expert, your role is to provide clear, concise, and informative responses to finance-related questions. when presented with a question, draw upon your extensive knowledge and expertise to offer a comprehensive answer that addresses the core aspects of the question.
system
what is a margin-call and how are they enforced?
user
if you don't have a margin account, then you will not have margin calls. you need a margin account if you wish to "buy on margin", to sell stocks "short", or to sell options, or maybe some other esoteric things i have not thought of.  if you don't do those things, then you do not need a margin account and will not get margin calls. in your example, it doesn't sound like margin has been used, if you deposit $20 and used it to buy $20 of stock and it then falls to $5, "they" did not lose the money, you did.  but if no margin was used, then no margin call would result.
assistant