Keep in mind that on 9/11 much of the US still had AMPS cellular service, and there were many dual mode AMPS phones about. AMPS is now completely deactivated in the US. Some countries never had AMPS, and began service with GSM. AMPS had longer range and better coverage than GSM.
If people were able to actually place cell phone calls from UA 93, it may have been over AMPS (although one mention way back in this thread suggested they used a special aircraft phone service).
I accept this, but one is not looking for a connection long enough to make a meaningful phone call, just a connection long enough register on the network (not even with the recipient) that a call had been made. Certainly, and obviously this is in a horizontal rather than a vertical plane,there are problems in the Dover, England area where callers connect to French mobile networks in error where the French signal is at least 22 miles away. Yet there is no suggestion any calls were made.