Hi! What a great question! Phrasal verbs can be difficult but with a little practice they become very natural sounding. You must understand them first though. So let's look at your examples: Call on- We usually use this when we talk about needing someone, not in reference to a short visit. For example: I am upset and need someone to talk to. I can always call on my best friend. She will be there. Call over- We use this to say we are asking someone over (could be for a short visit to have a conversation). They are not currently with us so we call them over. Mostly used in person. For example: At a bar- John is so handsome....and he's single...you should call him over (meaning he is at a different table and you want him to come to yours). Drop in- Definitely used to talk about going to a person's house for a visit. Usually this might be unannounced. For example: I haven't seen Joan in weeks and we are in her neighborhood. Let's drop in and say hello (can also say drop by). She told me I could drop in and see her this afternoon (example when the person does know). We don't say drop in at ________. It's mostly likely followed by an infinitive. We should drop in to see Sally. We should drop in to check on Gorge. Also common would be followed by a conjunction. We should drop in and check on Sally. We should drop in but I don't think he's home. Come by- This one is most common. We use it to mean come to where I am and have a visit me. For example- You can come by my place when you finish work. You should come by next week to see the new house. When are you coming by to visit? It's been too long. All these tips are for American English. Good luck! Let me know if you have any other questions. :)