Putting aside considerations of Can I/May I, I think possible translations of the phrase are "Can you spend some [significant] time with my concern?" or "Are you free right now?", or even more simply, "Knock, knock." It's not really a guestion, more a ritualized greeting/introduction like "Hi, how are you?" Responses addressing only the words are all going to be awkward in one way or another, as (being off the subject) they should.