T-Mobile in my opinion. My wife and I both have T-Mobile service and get GREAT coverage/reception all around town, on the mountain, and 75% of the drive on HWY 395 to/from L.A.. The answer is different depending on who you ask so we are asking...how does YOUR cell service work in Mammoth?
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A little more personal insight...I had a Verizon phone and it worked OK in Mammoth but experienced random dead spots (a spot worked 20 minutes ago and now it does not). I currently have an AT&T work phone which works well but only when you find the right spots in town (once you find the spots you are solid). No matter how much my friends complain about dropped connections with their iPhone (AT&T), they still love their iPhone and say the features outweigh the connection issues. I have not heard anything on Boost or Alltel. I hear Sprint is much better than they were four years ago.
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My Iphone 4 works great. I have service almost everywhere. Even in Clocktower. Never used to get service down there. It works great on the mountain as well. Its only bad when there is a mass amount of people in town. I.E. Christmas and New Years. Internet is a little slow unless you have Wi-Fi.

That really depends on what time of the year, oddly enough. If you are here on holiday weeks, AT&T and Verizon will have alot of dropped calls and poor reception, just because of the sheer number of people in town all trying to feed off of the same tower. When town isn't busy, there two servers get great reception. I have T-Mobile as well, which works better during the busy holiday weeks (probably because of fewer people using this service) but in general gets much less reception. Most places in town I get reception fine, but the second I head north of town, I loose reception pretty much until hitting the Nevada state line. Yosemite, June Lake, even Red's Meadow Resort, and Convict Lake, south of town, all have absolutely no reception for Tmobile users.