Gravity Falls - Dipifica: Mabel Approved.

Explanation A: Dipper doesn't want a cell phone because of paranoia.

Explanation B: Mabel asked for one and Dipper didn't.

Explanation C: Mabel was able to annoy Stan enough to get her a cell phone.

Explanation D: Maybe the cell phone in question doesn't even belong to Mabel. Maybe it's Stan's cell phone and Mabel took it for herself.