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One may not speak in a bathroom 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.
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.
May one talk in a Mikveh or bathhouse?
It is an act of piety to avoid speaking in a bathhouse or Mikveh unless it is absolutely necessary.
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:
The Soles Belula writes it is forbidden to speak in a bathroom in order to prevent the Kelipos from nurturing from one’s voice, as the speech of a Jew is holy.
 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 Oar 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.
 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”.
 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; Daas Torah 85; Darkei Chaim Veshalom 357; Shulchan Hatahor [Shomer Emunim] Tznius 4; Minchas Yitzchak 4/61
 This is because a Jews speech is very precious and there are Kelipos which nurture off this speech. [ibid; See Piskeiy Teshuvos 85 footnote 6]
 Shulchan Hatahor [Komrana] 3/3; Oar 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; Oar Ganuz 2/19.
 Shulchan Hatahor ibid
 Sefer Chareidim ibid
 3/2 brought in Kaf Hachaim 3/6