When people use their phones in public washroom stalls, thus forcing others to wait needlessly, is this not most selfish and inconsiderate? ?

When there is no one waiting to use the toilet stalls then feel free to use them at length. But when people are waiting, common courtesy requires that we make our own time in the toilet stall brief, that activities we can easily take care of outside stall be taken outside the stall. 

I once had to tell someone they were in a toilet booth, NOT a phone booth!

That must of rattled them because the next thing I heard was "Oh chit, I dropped my phone in the toilet!" I left the bathroom!

i think its selfish too

Just join in the conversation with them. It drives them crazy. 

I've never run across anyone using a restroom stall as a "phone booth."

Do you really need to ask this question, or are you just terribly bored?

Why wait for what?? When their done they need to get the ***!! Out