ON HIS VACATION

My boss will be on his vacation from .... to ....
If you have any urgent matter, you can reach him at his mobile number
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My boss will be on his vacation from....to.....
If you have any urgent matters or message that must tell him , (you can) please contact him in xxxxxxxx.


If you have any urgent matter, you can get him through by calling his mobile.
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If you have any urgent matters , you can call him through by his mobile number.



Therefore, I suggest the first one.