It's all going to depend on what service you have.
I've had AT&T for years, and coverage in the park has steadily become better. I remember getting some spotty coverage as far back as 2006.
As of very recent years, with AT&T, I've gotten coverage at:
- Several miles east and west of PJ
- Some spots along the north end of the Basin road
- Almost the full length of 385 between PJ and Persimmon Gap
- Sotol Vista and a few high spots along Ross Maxwell
- Some of the Grapevine Hills campsites
- Santa Elena parking lot
- The northern section of Old Maverick (starting about where the Rattlesnake Mtns campsite is)
- The last few miles of 118 on the west end of the park
The coverage varies. Sometimes I can send and receive texts. Other times I can only receive texts. Phone calls are a total crap shoot. If I can get through, then it's usually dropped within 3 - 4 minutes.
I get total coverage (calls, texts, data) in Terlingua/Study Butte, but data is very slow.