Mike: Can you give me a call when you're free.
John: Sure thing. I can't talk at the moment as I'm in a meeting.
Mike: When will the meeting finish?
John: Not sure.
Mike: Approximately?
John: I really don't know.
Mike: Look I really need to speak to you tonight.
John: I'll try to call you but I can't promise anything.
John can't talk at the moment as he is in a meeting. John doesn't know when the meeting is going to end. Mike needs to speak to John tonight. John will try to call Mike when he's free.