Rude. Inconsiderate. Impolite. Discourteous, etc.
Mobiles are obnoxious. (the device itself isn't, but the person that uses it may be). Well, there are certain ringtones that are, indeed, obnoxious. In spite of what way too many people seem to think, mobile phone conversations are not something that everyone else should have to hear. That is what texting is for.
Emergencies are different (obviously). If one is experiencing an emergency of some kind then fine. But two people having a conversation about what mascara one should purchase, how many calories one consumed that day, how ones date went the night before, why one can't stand her mother-in-law, how bloated one feels, etc.. Those are all p-e-r-s-o-n-a-l phone calls and the person should take it outside.
Most medical facilities, doctor's offices, medical labs, hospitals and care centers post signs instructing visitors or patients to not use their mobile phones, laptops, tablets, etc., due to interference with medical equipment. Signs are also posted on school campuses, libraries, police stations, government offices, on some public transit, trains, in taxis, in courthouses, churches, in museums, airports, all security check-points, in many retail stores, auditoriums, concert halls, restaurants, private clubs, or any production or recording studio. There are many situations where mobiles are simply not allowed and if one were to use one, personnel would either confiscate the mobile (returning it when the person leaves), the mobile-user would be sent away to store the phone in their vehicle, or the person may be asked to leave the premises.
As far as the hypothetical in your question, most people are aware that using a mobile in a doctor's or dentist office isn't allowed. They aren't even allowed in veterinary clinics. Using a mobile in a medical facility (where scanning and other medical equipment is being used) is absolutely prohibited. There is virtually no (good) reason why the mobile-user would have continued to stay in a small waiting room, and carry on an entire conversation whilst others are sitting there. As if everyone there is interested in hearing someone's personal conversation... Not to mention the fact that (presumably) people are waiting for the Dr. because they require medical care. Anyone requiring medical care probably doesn't feel that great. Having to listen to some chatterer drone on is asking too much.
The other patients shouldn't have had to ask this person to take it outside. Unless she was raised by wolves in a barn, under a rock.. she should have known better. Too bad none of them said something like "..my pacemaker is about to short-circuit because of you're mobile phone use".. (not that that is necessarily true), but anyone that clueless would never know that.