The calendar in MS Teams is merely a reflection of the user’s calendar in Outlook (Exchange), so there isn’t much to manage there; whatever they have in Outlook will appear within their own instance of Teams.
Task management in MS Teams is best handled by MS Planner using the Tasks by Planner app (added to a channel as a tab within the top menu of any given channel). This is best used as a project management tool where users can be assigned to a task, urgency and time tracking associated, lists and checkboxes (steps) can be added, and files can be attached to the task for quick access to pertinent documents.
Chat is two-fold:
1. The main chat area is where one-to-one chats might happen internally or externally with vendors or clients.
2. Also, team members can have group chats with their team members within channels; that is, each channel in a team has its own chat space where all members of that team and channel can discuss a specific topic.
Of course, Teams also offers audio and video calls, as well as conferencing, so there are those communication methods. But MS Teams can also be your Phone System using Public Switched Telephone Network (PSTN) connectivity with your existing numbers that might be ported over to this service or new lines/numbers for specific users or service numbers (resource accounts) for call queues or auto attendants.