Has LE ever confirmed (or has it been reported) that Jayme has her own phone? I recall LE stating that they had recovered the phone that placed the 911 call, and the release of the dispatch logs strongly suggests that the recovered phone was Denise's (although she may or may not have placed the 911 call). If Jayme does have a phone, I'm not certain they've recovered it, which means that they may only be working with anything backed up on the cloud or synced to a computer or another backup device.
I'm not certain James and Denise would give Jayme her own phone fresh out of the gate. Most couples I know with kids start with sharing devices to see how responsibly the kids use it. I know we've seen many photos of Jayme with a phone, but could that have been her mother's phone or a device that would work with wifi, but without cell service (a Touch or an old phone)? I bet that someone in her life would see fit to give her a hand-me-down device at some point, and if it's a wifi-only device, it would make her electronic footprint more difficult to track.
If her parents gave her a phone/device, they likely monitored her use ... but I'd also guess that Jayme's friends would know how to circumvent monitoring, or she could easily google it. That would make Jayme a normal 13-year-old.
I'll also mention that sometimes kids who are shy and quiet in person find it easier to communicate online / virtually / electronically than they do in real life.
