If he got a new phone number with the new phone on September 4th, then Gabby's mom would have been trying to reach him via texts and phone calls to his old number.
And based on not being able to get ahold of either of them on the phone #s she had, she would have every reason to be extremely concerned and to alert the Laundrie family that perhaps BOTH GP and BL were missing.
Again, the lack of response by the Laundrie family to this very alarming situation is unfathomable. If I were GP's mother and did not have a response from either of them, I would likely be contacting LE in the last place they were known to be. I believe that would have been Grand Teton National Park, as GP had done the video chat with Mom on 8/25.