Cell phone calls from airplanes

A very smart, well-read, highly-educated friend of mine is convinced that the "standard explanation" for 9/11 is a hoax.  As one proof, he points out that you can't make a mobile phone call from an airplane in flight, like supposedly the passengers  of United 93 on Flight 93 did before the plane crashed in Pennsylvania.  That famous "let's roll" from heroic passenger Tom Burnett?   Never happened, he says.

So in the interest of debunking the conspiracy theory, I risked the ire of the flight attendants during my plane ride this weekend, turning on my AT&T Tilt in order to test the signal strength.  I figured I'd call my friend from the plane, maybe leave him a ha-ha-guess-where-i'm-calling-you-from message on his answering machine.  Yeah, yeah, I know they ban cell phones out of fear that they'll interfere with "sensitive communications equipment", but the New York Times says there's no evidence for that, and my experiment was for a good cause.

But guess what? It didn't work.  Try as I might, I was completely unable to get a signal.  It just wasn't there.  Only as the plane was on its final descent, not more than 5,000 feet from the ground, did the signal strength become strong enough to detect.

So what does this mean?  I can think of a few possible explanations: 

• The planes flew much lower than cruising altitude, at least when the calls were made.
• The passengers on Flight 93 didn't use cell phones -- they used those in-flight GTE Airphones.
• Passengers were using much older, analog phones that have more signal strength and work on a different technology that maybe reaches the ground better.

What do you think?  I'm not a conspiracy theorist, so those are the only explanations I'll suggest, but I admit it does make me curious.  Have you ever tried successfully to make a phone call from a plane?