ASBI Pocket Beit Midrash: Aruch HaShulhan, Laws of Synagogue 151:3-4

And one should not engage in financial calculations inside of a synagogue or beit midrash unless they are calculations that pertain to a mitzvah, such as calculations of a tzedakah fund or for redeeming captives etc. And there should be no eulogizing in a synagogue unless it is a eulogy for a great person. Or, if the eulogy is relevant for an influential resident of the city such that many residents fo the city will gather to pay tribute to the deceased and the entire community will gather in honor of the bereaved relative...

And what should one do if one needs to enter into a synagogue or beit midrash, for example to call someone from there etc.? One should enter and then recite a few words of scripture or some halakhah and only then call to one's friend so that it does not appear as though one entered the sacred space for his own mundane purposes. And if the individual does not know how to recite scripture or learn a halakhah, he can turn to a child and say "read for me the verse that you are studying now." Or, he can wait a few moments in the synagogue or beit midrash for merely sitting there is also considered a mitzvah as it says "happy or those who dwell in your home." And then the person can call to whomever he wishes. And how long should he sit under these circumstances? Enough time to walk from one entryway to another as I wrote above in Siman 90.

And one does not have to sit for standing is the same and "yeshiva" which literally means sitting here means being detained and stationary as it says "like the days that you dwelled." But one should have in mind that sitting or standing one is fulfilling the mitzvah of dwelling in a synagogue or beit midrash. And it seems to me that in this one one could also enter a synagogue or beit midrash for shelter from the sun or cold or rain. And even if it looks as though one is entering the synagogue for protection from the elements, nonetheless, if one then learns a verse or a halakhah, it will not appear so flagrantly as though one entered because of the rain, sun, or cold.