They could have different providers. I used to camp in an area similar to Timber Creek, just a little further east, and most of the time I was the only one to have cell service. I had a different provider than the others.
We don't know that EVERY call she made to Trina went through, just some. We don't know that JM was able to complete every call she made. We only know she was able to get at least one (or a few) calls to Trina and one to 911. VDK driving down the road to get a signal isn't a red flag at all.
If you were in an emergency situation, and you state in your post you've been in those mountains with uncertain cell service, wouldn't you want everyone with you to do whatever possible to get a call for help? Even if it meant "hauling" down the road? Wouldn't you want to talk to your mom, dad, husband, wife, sister, brother, or whoever it is in your life you need? I have adult children, all living on their own with families of their own and when something bad happens in their lives I, their mom, is one of the first calls they make.