Service is intermittent and patchy at best regardless of carrier, I use Verizon and have had a usable signal at a number of different places throughout the CG but never rely on it being available. Fortunately, one can usu pick up a decent signal just a short distance down the road... I've had multiple bars at Fern Basin, for example.
I camp there regularly. My experience is cell service is less than spotty. However, on site #8 about ,10-15 meters to the south-west facing Hemet, you have reception. We use TMobile and last week it worked flawlessly at that location and has for several years.
I was just there and had spotty service with Verizon. We were able to make calls when we needed to though. The town of Idyllwild just down the road had great service.
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