Cell phones can be quite cheap so adding one to a handheld like the 3DS wouldn't raise the cost too much.
Okay, so there have been primarily videogame handhelds that also incorporated cell phone technology in the past and was either unpopular or just wasn't functional properly as both a handheld and a cell phone. With cell phones being incredibly popular, there shouldn't be any reason in future why the two remain seperate.
But of course, you want a dedicated cell phone, it'd be better to buy one specifically for doing so. Likewise, you want a proper gaming handheld then you stick with Nintendo's.

