It's a me! Mario!
ON24.COM recently reported on the rumor that Nintendo is rumored planning to incorporate cell phone functionality in it's upcoming Game Boy Advance. What is interesting, was that the story also stated that Nintendo may start selling adapters before the arrival of the GBA that would add cell phone functionality to existing Game Boy systems ...
ON24.COM also stated that this is unconfirmed by Nintendo, at this point.
It is also unknown, and perhaps unlikely that the US market will see these features. Cell phones in Japan are much more common than in the US, as their usage in Japan is actually less expensive than using traditional land lines.