The one difference why a GIRL would be worse off having a cell phone is that people tend to talk much louder on a cell phone, broadcasting their conversation to all unfortuante enough to be in their proximity. When it comes to a girl, it is plainly not tzniusdig, especially when acompanied by shrieks and loud laughter.
That being said, for some reason ppl think that if you text someone it is not the same as having a conversation with them- wrong!! I know girls who would NEVER (sarcastically said) talk to boys, yet see no problem w/ dropping them a message or two.Even if that doesnt lead to going off the derech, it stills invites arayos and is therefore assur.