McGraw-Hill's Essential Phrasal Verb Dictionary - Richard Spears

Phrasal verbs, also called two-word verbs, are idiomatic expressions wherein the second element of the verb (the adverb or particle) is not necessarily predictable. For instance, why the word up in call up a friend? Why not say call on a friend or call in a friend? Actually, those are three separate, unpredictable combinations, and they each mean something completely different. For example, you can call up a friend on the telephone, call on a friend to visit a friend’s home, and call in a friend to come help you with something.