This is another major issue. I found the following about it, it's a post taken from a site banned by DU, but you should be able to find it if you google any of the phrases in "".
"It is possible that calls made from the hijacked planes on 9/11 would have a better chance of getting through to 911 than a new phone would in an emergency today.
"I worked in the marketing department of a US cell phone company in 2001. At the time, Tri-Mode phones were standard. Tri-Mode phones received 2 types of digital as well as analog signals. By the time the company released its first digital only phones, many who previously enjoyed using their cellphones upgraded to find that they were unable to get a signal or maintain a connection. The following explains why:
"From "Will your cell phone reach 911?", Consumer Reports, 2/2003
"When your phone is in digital mode, it can work only with your home carrier (the company you use for service) for any call--including those to 911--unless the home carrier has a roaming agreement with another carrier. Phones that can work in both digital and analog modes give you more options. Analog provides that safety net for emergency calling. Indeed, the principal FCC regulation governing wireless 911 recognizes the importance of the analog mode. The regulation, which took effect in 2000, says that whenever a wireless phone dialing 911 in analog mode can't get through via its home carrier, that phone must seek another signal, even if it's from a competing carrier, to quickly establish a voice connection. The FCC concedes its rule is only a small step toward improving 911 service. Multinetwork phones, which are normally in a digital mode, aren't required to switch to analog to make a 911 call. There are no regulations for digital-only phones..."
"In other words, for phones trying to make 911 emergency calls using analog, cell carriers were required to seek out the nearest tower (even a competitors' tower if necessary) to quickly connect the call. There were no such requirements for phones attempting to connect to 911 using a digital signal. This actually did make a phone with analog a better bet for reaching 911. However, companies began phasing out analog, passively at first, after the FCC lifted requirements that they maintain the analog system in Fall of 2002.
"Among Consumer Reports conclusions:
"The FCC must ensure that digital phones are more compatible.
The FCC voted last fall to phase out its requirement that some wireless providers offer an analog backup signal. We think that was a mistake because the agency did not also require companies to make their digital technologies talk with one another. Simply allowing analog to fade away removes the principal common wireless language. In the end, you will have less assurance than you do now that your phone will get through to 911.
http://www.consumerreports.org/main/detailv2.jsp?CONTENT%3C%3Ecnt_id=299615&FOLDER%3C%3Efolder_id=162691
"So attempting to make a call from a plane today using a newer technology cell phone isn't really a fair comparison. The analog system is patchier and discontinued in many places altogether; many phones only offer access digital now anyway. On 911, the callers on the hijacked craft were almost certainly off the digital network using a trimode (or lower quality) phone. When digital couldn't get through, their phones switched to analog which, at least in a 911 call, gave them a better chance of getting through.
"This still doesn't clear up how calls were made at altitudes over 8,000 ft (and possibly up to 30,000 ft).
"Thanks,"
I didn't know this. If many of the passengers really did have phones that would automatically switch to analogue mode, then it makes the calls a whole lot more likely.