What if Brittanee lost her phone, or someone stole it? Maybe she was looking for it and couldn't text or call anyone whenever she was taken? Her phone could have gone in one direction while she was going in another. Maybe this has been discussed before, but I'm just trying to think of different angles.
I suppose anything is possible, but it seems unlikely that she lost her phone or had it stolen right in the middle of a text conversation with her boyfriend. Her communication with him abruptly stopped, which is what caused him to become concerned in the first place. I wonder if she was grabbed from behind as she walked, and dragged into a vehicle or forced into its trunk. She may have dropped her phone when grabbed, or was forced to turn it over if her kidnapper had a weapon. In the heat of the moment the abductor may have slipped her phone into his pocket and then sped away, forgetting to turn it off. I think the phone was ultimately ditched in the swamp, possibly not too far from her body.