I'm confused in general as to what on earth women are actually doing in the stalls. I get so frustrated waiting ages in line because women take so long. You can often not hear any bathroom type noises for 5-10 mins at a time from a stall before someone emerges - what is that about? I know sometimes there's other stuff that you might want to do in a stall - adjust clothing, menstruation related things - but it feels like almost every woman takes ages and they can't all be doing this stuff. When you know that there's a queue, I think that's really rude, whether or not it involves a phone!
I know once I had the most awful flu. It took me down for 4 days. On the third day I gained some strength and had to go get a perscription filled. I walked into Big Bullseye Store, felt sick, got dizzy, headed to the bathroom and almost passed out. I was in there for probably 20 minutes before I was able to get back up. It probably looked like I was sleeping or something. I'm a get-in-and-get-out type of person, but after that, I don't think I'll pass judgment just because someone wasn't making bathroom related noises. Unless it's a one stall place, I don't think it's rude to take the time you need in a bathroom.
Maybe that's the crux of it for me. A stall had opened up, it just wasn't the one the OP wanted to use so she chose to wait. It may have required a lot of manuvering, but it was possible for her to use it. So if there was a queue, one bathroom was open and available for use.
I do think it would be OK, after a few minutes, to politely tap on the door and say that you need to use *that* stall. It alerts the person that there is a queue and you cannot use the other one. My handicapped Aunts have done this on a few occassions where someone was in the handicap stall for a while.