There's simply no substitute for Kanban processes and for proactive communication from engineers. In a small team without dedicated customer support, a manager takes the customer call, decides whether it's legitimately a bug, creates a ticket to track it and prioritizes it in the Kanban queue. An engineer takes the ticket, fixes it, ships it, communicates that they shipped something to the rest of their team, is responsible for monitoring it in production afterwards, and only takes a new ticket from the queue when they're satisfied that the change is working. But the proactive communication is key: other engineers on the team are also shipping, and everyone needs to understand what production looks like. Management is responsible for balancing support and feature tasks by balancing the priority of tasks in the Kanban queue.