Talking while in the bathroom
Is one permitted to talk on the phone while in the bathroom (with someone else who does not know the caller is in the bathroom)?
One who enters a bathroom to relieve himself may not talk unless it is extenuating circumstances. When he is relieving himself even that is forbidden, see Shulchan Aruch O:C 3:1 and the Mishna Brura there. This would apply to talking on the phone as well.