140. Calling a Someone Who Isn’t There

23:11 If a kohein or Levi was called for an aliyah and it turns out that he is not in shul, his "understudy" should not be called by name so that people shouldn't think that there is some defect in the one originally called. Rather, the substitute is simply told to go up without being called by name. The son of the original kohein or Levi may be called by name because this will not mislead people; if the father were unfit, the son would also have been unfit. Similarly, if the kohein or Levi could not accept the aliyah because he was saying Shemoneh Esrei, his replacement may be called by name since everyone can see why the first person did not come up. If a Yisroel is called but is not present, his replacement may be called by name since there is no concept of a disqualifying defect in a Yisroel. Similarly, if a kohein or a Levi was called for the final aliyah or for maftir and he was not present, another person may be called by name (even another kohein or Levi – MB 135:41).

23:12 What if no kohein was present in shul, or if one was present but nobody knew it, and a Yisroel was called for the first aliyah, and then a kohein arrived or others realized that one was present? Even if the Yisroel already said Barchu, so long as he has not started the bracha "asher bachar banu," the kohein takes his place and says Barchu again. In order not to embarrass the Yisroel, he is to remain standing at the bimah and given the appropriate aliyah. However, if the Yisroel has already said God's Name in the bracha, the kohein does not take over the aliyah. The same applies if a Levi was called in lieu of a kohein. This is also the case if no Levi was present or if the others didn't know one was. If the kohein was called in the place of the Levi, and the Levi arrived before the kohein says God's Name in the bracha, the Levi is called for the aliyah. (In such a case, the kohein does not stand by for another aliyah like the Yisroel does, since he already had one – Mishnah Brurah 135:26.)