First appearance (Usagi): Act 1
First appearance (Ami): Act 3
First appearance (Rei): Act 6
First appearance (Makoto): Act 9 (although she probably used it in Act 8)

As well as being used to communicate, if someone uses one of these cellphones to take a photo of someone, they can be disguised as the person in the photo. It's possible either to to transform their clothes straight away (as Usagi did in episode 1) or keep the photo saved and transform later (as she did in episode 2).

Luna gives Usagi and Ami their Teletia-S cellphones. It can be presumed that Luna also gave the other Guardians their cellphones.

Anime/Manga Comparison:
In the anime and manga, Usagi has a transformation pen that she uses to disguise herself. Instead of taking a photo, she has to shout "Moon Power!" and say who she wants to disguise herself as. Personally I think the cellphone is much cooler, even if it is bright pink. Ami and the others don't have transformation pens or cellphones in the anime or manga.