Common Sense Phone Ettiquette

So my brother's half sister T posted on FaceBook that she was talking to a friend on her cell phone in the Sauna at her gym. A woman came into the sauna and rudely demanded that she end her conversation as it's rude to the other users of the sauna.

Because she demanded instead of asked, T told her no and that there's no signs prohibiting her from cell phone usage.  Needless to say, a fight almost ensued.  

What's so interesting about this is that at no time did T think she was being rude.  BDDS...(Break Down Dizzy Style):

Since when does a fucking sign need to be posted outside a gym sauna saying "No Phones"? Really?  One would think common sense would kick in here and say...

A.  Everyone can hear my conversation, and

B.  Everyone may not want to hear my conversation.

Places where I'd think twice about talking on my cell would be... Sauna's, locker rooms, bathroom stalls, restaurants and movie theaters. I know there are more areas, but why do you need a sign to tell you these things??? REALLY???

What makes your fucking call so important that I have to listen to it?  Oh yeah, I forgot...the world revolves around YOU!