The ringing of cell phones is rude. It is disrespectful to everyone else in the room as it intrudes on their ability to listen and/or participate in the activity without unnecessary distraction. It is disrespectful to the person speaking as it sends the message "You are not important enough to receive my full attention." It reflects badly on the offender and is a poor representation of their company; is he that unorganized that he can't remember such a simple thing as turning down his phone? Does she think she's so important that her needs should come above everyone else?  

For the record, when I am speaking to a group, I don't make a request for people to turn off their cell phones. For one reason, I usually forget. Second, and more importantly, I assume that anyone in business or employed with any level of responsibility is also mature enough - capable enough - to do so of their own accord, without a reminder from me. In my seven and a half years of speaking and teaching, it has never been a problem.