And yes airphones.. I remember there being sever calls reported..
There were several calls, as I reported in my previous post, but this wasn't a very efficient method of information transfer compared to what you could do today. As
@wcv215 notes, it depends on people onboard the aircraft calling out and getting to people who know what's going on. Not all that difficult, but it still introduces a certain degree of lag time between each strike and when people on board the other aircraft could learn about it, which is borne out by the actual call records. Only on Flight 93 was there a sufficient gap between when the attacks started and when passengers learned about them that they could attempt resistance effectively, which is borne out by the fact that only on Flight 93 did the passengers actually attempt to retake control of the aircraft.
For example, the Flight 175 passengers and cabin crew don't seem to have learned about Flight 11, the previous attack, at all, and were nevertheless considering trying to storm the cockpit--but only a few minutes before the aircraft hit the World Trade Center, far too late to do anything successfully. Similarly, at least one passenger on Flight 77 did learn of the attacks from her husband, the then-Solicitor General, but only about 10-15 minutes before the aircraft hit the Pentagon. Trying to retake the aircraft would have required that she persuade the other passengers that it would be worthwhile, then organize an attack on the hijackers in this extremely limited time window. Possible, theoretically, but very unlikely.
There's a reason I spent an entire paragraph discussing exactly how information flowed, or rather failed to flow, to the hijacked aircraft and how this meant that there was just not enough time outside of the old paradigm for any kind of resistance to form. It's kind of annoying you totally ignored that paragraph, which I wrote specifically to forestall this argument.