Translation:If you have questions, call me personally.
The English correct translation has added one word "just" which is missing from the German sentence. "ruf mich persönlich an" vs. "just call me personally".
Which German word could be used for "just" in this particular sentence? gerade; genau; nur; gerecht; einfach; eben; soeben; .... ???
What's the difference between this German sentence and "Wenn Sie Fragen haben, rufen Sie mich persönlich an"?
du is used to speak to one person informally; Sie is used when you're being more formal (e.g. to a customer), and can be used to one person or to several.
As for falls versus wenn -- falls can only mean "if", while wenn is theoretically ambiguous between "when" (definite future) and "if" (hypothetical future). But in this sentence, I would understand it as "if" as well; falls and wenn are synonyms here.