There is not enough information in this verse to really understand the nature of Nephi's means of assuring his brothers. One may assume that simply calling to them was insufficient, for although it would likely have been Nephi's voice they heard, they would likely have misconstrued it on the basis of their assumption that it would be Laban's voice. Certainly the telephone has taught many of us that even the most familiar voices can be difficult to recognize when stripped of their context.