I'm currently in the area of Mammoth Lakes, in fact camped off Owens River Road. I may have to move though as smoke and ash from the Lions fire, southwest of Mammoth Lakes, is really bad right now as the fire grows in size. The fire is only 14 miles away from here so the air quality is terrible.
That being said... there are many beautiful free campsites in the area. I think you will find good signal all along the 395 corridor but as soon as you venture any distance off the highway it will be iffy. As Blanch said, there is good boondocking all along Owens River Road, where I am I have 4 bars of Verizon. Also there are lots of good campsites along the Mammoth Scenic Loop Road with very good cell signal.
Some free campgrounds in the area, Glass Creek CG is close to the highway and has great signal but it is a very popular and busy campground. Last week I rode through and didn't see an empty spot. Big Springs CG is just up the road, I didn't check signal there. Hartley Springs CG is further off the highway and has zero cell signal. I would guess the same for Upper and Lower Deadman CGs as they're even further off the road. Laurel Springs, on the other side of Mammoth Lakes had weak Verizon but good AT&T. Further south, Crowley Lake BLM campground had really weak signals.
Its about 20 miles up to Lee Vining and the turnoff to Yosemite. As you go north there are boondocking opportunities but they are fewer. As long as you're near the 395 signal should be okay. I do remember that once you leave 395 and head up Highway 120 toward Yosemite there is virtually no cell signal.
