Yes, yes it is. A notable scientist conducted an experiment, as shown in the film. It conlcuded that the likely hood of a successful phone call goes down exponentially as the height of the plan increases. I think I remember the chance of a successful phone call being made was less than 1%. I think you can agree that the chance that all of those were successful were quite low. Now I know you aren't going to take my word for it so take these:
http://www.rense.com/general56/cellpp.htm
http://911research.wtc7.net/planes/analysis/phonecalls.html
http://www.globalresearch.ca/articles/CHO408B.html

There are many others but I would like to point out a few key lines:

I would also like to point out some more information that I have found in my popular science magazine this month. Cell phones constantly monitor their psition when you turn them on. When you attempt to call someone the phone sends its data as well as the number you dialed to the carrier's nearest tower which sends it to the MTSO. Now these MTSO's are not located at 10000 feet. They are very limited in range. In my town we have terrible reception. You normally have to walk outside to get a signal. AND the nearest MSTO is less than a mile away. Now you are going to tell me that people were able to make calls from within an alluminum tube, much further from an MSTO than me, while traveling over 200 mph, and all of them have perfect connections? To me that's seems a little unlikely.

Edit: @Jon if they thought it was a traditional hijacking, THEN WHY DID THEY GO TO THE TROUBLE OF TRYING TO OVERTHROW THE TERRORISTS?!?!? I remember watching a show on these calls on the History channel a while back. Many of the callers mentioned that they knew of the attacks earlier and they weren't about to become the next victims of a suicide attack.

@Chipmaster: The callers obviously were able to keep their heads on their shoulders. They most likely believed it was a traditional hijacking, in which the terrorists hijack the plane, land it, and relase the hostages after a while. They probably figured out that if they called their loved ones and screamed their heads off, nobody would know what was going on and it would just cause their family members undue stress.

But in the air, there is no terrain, no trees, and no buildings to block signals. There is not as much interference when a plane is directly above a tower as opposed to a person on the ground a mile away from it.