I've been able to make phone calls in one of the campground parking lots between Fish Lake and Johnson Reservoir. There's also some spots as you get up Seven Mile canyon above Johnson reservoir where I've been able to make calls and pick up enough data to do more - including use my phone as a hotspot and jump online.

There used to be a cell phone booster at the little store there at the Fish Lake marina - I'm not sure if it's still there or not.

Honestly - most of the places I've had cell service and data have not been places I want to hang out - unless I'm on top of a ridge glassing for animals. But then you aren't at the lake anymore.