"Call upon" is a multi-purpose verb that has various synonyms depending on its context. The most common synonym for "call upon" is "request" or "ask," which implies making a formal appeal to someone. Another suitable option is "invoke," which denotes using a higher power or authority to summon assistance or action. However, "call upon" may also mean visiting someone, in which case "drop in on" or "pay a visit" may be suitable alternatives. Furthermore, if the context involves summoning someone to perform a task, "invite" or "enlist" is more appropriate. Finally, "refer to" or "resort to" can be used when seeking clarification or advice.