mempilot,
I can't vouch for how well cell phones work at altitude but modern cell phones definitely work at ranges greater than 6 miles. In congested areas (e.g., cities and suburbs) the towers are directed horizontal and down to limit ranges but that isn't the case out in the open areas like the deserts in S. CA.
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Cell Phones will work at different altitudes depending on the phone, distance, which technolgy is used (CDMA, etc). and how directional the antenna is (in design).
Remember, just because you're 6 miles up, doesn't mean your directly above the tower. Chances are it won't work 'hovering' 6 miles up directly above the tower because the antenna isn't designed to be directional "straight up".
Also, if you're 6 miles up and 6 miles (ground distance) from the tower, then you're 8.49miles in distance from the tower.... (straight line).
It's definately workable for cell phones to work at greater distances that 6 miles. However, add altitude in that with antennas that are pointed in parallel with the ground, and chances are that you'll have diminishing returns of it working the higher it goes.
As for cell phones working at long distances.... I've used a cell phone at wreck sites that were 12 miles offshore before. I won't say we didn't have static and drop the call a few times, but it worked (with poor quality).
I also got calls from the site of the sinking of the USS Oriskany. Now that call got dropped I think 5 times in 10 minutes or so, and was poor quality, but it's 22 miles SE of Penscolsa pass. I don't have a direct compass heading to determine direct distance to the nearest shore point, or the nearest tower on shore, but I guess it was a good 18 miles "straight in" to the closest point on shore. Now in this case one provider worked but another didn't as someone else on their boat who used a different cell company, that had different technology and different tower location most likely, couldn't even get a signal and got 'service unavailable'.
So I think there are a lot of variables in determining how far a cell phone will reach.
BTW... just because it's on Wikpedia doesn't make it fact. Anyone can edit that and put in there any 'fact' they want. I've seen lots of errors on Wikipedia on many different subjects.