I'm not sure I'm following you... Are you suggesting the phone is in a package that was mailed/shipped/couriered to Gooding, ID? If that is the case, why did it not ping (as far as we know) along the way - anywhere other than Gooding? A phone can't turn itself on (yet).
Maybe it was off, until a certain person got it in the mail. I have wondered this myself.
Did Kelsey have someone from her past that lives in Gooding, Idaho? Maybe a friend covering for her, hence the "possible" mail delivery of her phone to someone she trusted, because she was scared or threatened by someone and feared for her own life? It may not even be related to PF, it could be a student or former student, a co-worker, etc- she might have been afraid to get her loved ones and friends involved, fearing for their safety as well as her own. Maybe she didn't want to contact them, to keep them safe.
I know, farfetched theory, but that phone ping and those two text messages are really bugging me. We're not talking an hour or two away, 10-20 miles away. Somehow, her phone is out there in Gooding, Idaho.
If she left on her own and is safe somewhere, then I hope and pray she contacts her Mom, at the very least. Still, it doesn't make sense to me, for her to abandon her home that has a mortgage, her vehicles, and most especially, her daughter! I wonder if she wanted to move closer to her family, especially since it doesn't appear that they are getting married anytime soon, and he said, "oh hell no, and if you leave, you aren't taking the baby with you!". All of the above is JMO and speculation only.