At the time of 9/11, cell phones did NOT work above 10,000 feet.
This has since changed. Cell phone technology has greatly improved.
I have argued that one of the calls from UAL 93 may have been genuine. The plane was very low. The call only lasted 30 seconds. A document from the 9/11 Commission released back in 2011 further supports the idea that it did happen.
However, most of the 9/11 calls are probably dubious. MHG