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Not to talk in a bathroom:
One may not speak in a bathroom [while using it for urinating or a bowel movement] as doing so is immodest. If it is not possible to [lock or] close the door of the bathroom, then if someone approaches him, he should grunt to him to let him know that the bathroom is occupied, although he may not speak to him.
Women: It is permitted, and even encouraged, for women to enter a [non-gendered] public bathroom together simultaneously. It is likewise permitted for them to talk in this bathroom. [However, by a private bathroom all the above laws apply equally to women. Furthermore, it seems that by a public bathroom designated only for women, as is common standard in all public areas of the modern times, the requirement for privacy and prohibition of speech would apply.]
Q&A on Talking
May one speak in a bathroom in a time of need?
If one has not yet begun to defecate, then in a time of need he may speak in a bathroom. If, however, he has already begun defecating he may not speak in the bathroom, even in a time of great need. [It is however permitted to talk of matters relevant to the bathroom such as asking for toilet paper, although seemingly even then it is best to hint to the matter rather than talk.]
May one speak on the phone while in the bathroom?
Based on Kabala, it is forbidden to speak in a bathroom at all, even if the other person is not present. Thus, one may not speak on the phone or even read aloud while using the bathroom.
May one talk in a bathroom if he entered for other purposes and does not currently plan to use it?
Some write it is permitted to talk in the bathroom if one does not plan to use it. This especially applies when the bathroom contains other usages, such as a shower, laundry or medicine cabinet.
Sparks of Kabala
Speaking in a bathroom invokes an evil spirit:
The Arizal warned that it is forbidden to speak even one word in a bathroom being that one who does so invokes the evil spirit called Tanya to reside in him. This spirit entices one to sin and causes danger.
The Kelipos nurture from the speech of a Jew:
It is forbidden to speak in a bathroom, as the speech of a Jew is holy and allow the Kelipos that are found in the bathroom to nurture from it.
 Admur Basra 3:2; Kama 3:2 simply states “One is not to speak there” as a continuation of the previous law and does not give further details; Rama 3:1 in name of Or Zarua.
Ruling of Admur in 85:2: In 85:2 Admur rules that one may speak of mundane matters in a bathroom. Vetzaruch Iyun, as this seemingly contradicts the Halacha here. See Even Yisrael 9:63; Piskeiy Teshuvos 3 footnote 17 suggests that there it is discussing one who is in a bathroom and is not currently using it in which case talking is allowed.
 See previous footnote
 So writes Admur in Basra ibid that the reason is due to modesty. However, the Mekubalim explain the reason is due to danger of the bathroom spirit, as will be explained in “Sparks of Kabala”.
 Basra 3:2l; M”A 3:2; Michaber Even Haezer 22:13; Sanhedrin 19a
Field bathrooms versus city bathrooms: Some Poskim do not differentiate between city and field bathrooms and imply the above ruling applies to all bathrooms. [See Michaber Even Haezer 22:13; Sanhedrin 19a] Other Poskim however limit this allowance only to field bathrooms, while by city bathrooms it would not apply. [Rama Y.D. ibid; M”A ibid] Admur clearly rules that it applies likewise to city bathrooms.
 Commonly referred to as a unisex bathroom which can be used by both men and women. See next footnote.
 The reason: The Sages instituted that when women go to a public bathroom, such as the public field bathrooms of previous times or public Shul bathrooms, they should enter two at a time or even more, and that they should talk to each other there. The reason for this is in order so the men outside hear the women’s voices and thus not enter the bathroom and transgress the prohibition of Yichud. [Admur ibid; Michaber ibid; Sanhedrin ibid]
 See Kaf Hachaim 3:4 and 5 that women are not to enter together into the bathroom. This is unlike the Peri Megadim 3 A”A 2 who learns this restriction does not apply to women.
 The above Halacha [see background] implies that the bathrooms in discussion were not gender segregated and hence if not for the women talking, there was possibility of Yichud occurring. [To note that gender segregated bathrooms did not exist in previous times and is a relatively new occurrence. For example, in the capital building, Washington D.C., one can see that there only existed one bathroom as of 1900, and only later was a women’s bathroom also installed] Thus, today that most public bathrooms are gender segregated seemingly this allowance would no longer apply, as there is no reason a man would enter due to it being socially inacceptable. This is similar to the ruling of the Rama [Even Haezer 22:13] and M”A [3:2] that in city bathrooms this law no longer applies, and hence concludes the Rama that the law is no longer applicable today.
 Shaareiy Teshuvah 3:2; M”B 3:4; based on Rambam Hilchos Deios 5 that prohibits speaking even in a time of great need. See Torah Leshma 24 who permits in a case of great need.
 See Piskeiy Teshuvos 3:3 in name of Salmas Chaim 10 [not found], however so seems logical.
 See Piskeiy Teshuvos 3:3 and “Sparks of Kabala” below
 Piskeiy Teshuvos 3:3; Yabia Omer 8:1 based on that in today’s bathrooms there are no longer any Mazikin.
 As in such a case many say the spirit of impurity does not reside in such a bathroom. [See Chapter 5 Halacha 4B]
 Shulchan Hatahor [Komrana] 3:3; Or Yisrael [printed 1700] in name of Arizal brought in Kaf Hachaim 3:6; Sefer Chareidim 47:7; Amudei Hashulchan 4:3
 See Beis Rebbe p. 106 in name of Tzemach Tzedek: “There is a Kelipa called Tanya which prevents one’s learning of Penimiyus Hatorah, as through learning Penimiyus Hatorah one abolishes this Kelipa. It is for this reason that the Alter Rebbe began the Tanya with the word Tanya and it is hence called Tanya.” This concept of the Kelipa called Tanya fighting against learning Penimiyus Hatorah is brought in Kisvei Arizal in the Pirush on the Idra; Or Ganuz 2:19
 Shulchan Hatahor ibid
 Sefer Chareidim ibid
 Soles Belula 3:2 brought in Kaf Hachaim 3:6