It is possible to get a cellphone to carry on Poptropica, but it is just decoration.
The actual cell phone is on Nabooti Island at the Mountains of the Moon. You have to climb up the trail, eluding goats and falling rocks (although the second goat helps you climb up). At the top, an old man in a snowy cave will play Mancala against you. Once you win, you can enter the cave and get the cell phone. (also the Red Jewel)
(see related question)