What is the difference between "reach out to", "drop in on", and "call on" in terms of meaning? Do they mean the same?
"Reach out to" can imply contacting someone who is not exactly close to you, you would use it for a professional acquaintance or someone in a professional context.
"Drop in on" implies calling or visiting someone in a casual context.
"Call on" is more formal, and it's not very commonly used for individual people. It could be used in the context of publicly asking organizations, institutions, societies or groups to do something.