'are' is the most appropriate here as the second part of the sentence gives the reader an impression that the employees have already left the office, and will be back only after July. 'will be' would have been OK, if the second part had been "any requests will be handled only over the phone"; in this case, the sentence would have given the impression that the speaker is letting people know "this will happen", and the employees haven't gone out of office as yet.
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