The phrase "calls on" can be replaced with several synonyms, depending on the context of the sentence. If someone is "calling on" someone else for assistance or help, the phrase can be replaced with "appeal to," "request," "ask," or "entreat." In a more formal setting, "summons" or "invokes" could be used. When referring to visiting or dropping by, "stop in," "visit," "pay a visit to," "pop in," or "swing by" could be used. In a more confrontational context, "confront," "challenge," or "accuse" could be used instead of "calling on." Overall, there are many synonyms for "calls on" that can be used to enhance language and add variety to writing.