We often have an actor in costume make the announcement, like I believe they do in certain Broadway shows such as Spamalot. For some reason, audiences listen more to actors.
During our production of Evita, we had a Spanish teacher say the announcement first in Spanish, then in English; it seemed to catch the audience's attention. Show-specific twists like that are always appropriate.
We usually say that cell phones interfere with the wireless mics, so it is imperative that they are turned off. We offer, like someone else pointed out, to collect the cell phones of anybody who must be contactable during the show and notify them of any calls. Audiences often listen to that.