There is too much confusion about what happens to your cell phone when placing an emergency call. Hopefully, I can clear up all the misconceptions.
Can I use any cell phone to place a 9-1-1 emergency phone call?

- Yes. As long as the cell phone has enough battery power and some tower reception. No SIM card is required. 
 
What if I don't have cell phone reception?
- When 911 is dialed on your cell phone and your carrier is not reachable, it will automatically search for any network/service to make the emergency call. Your cell phone will probably switch from digital to analog mode.
 
What if the cell phone is locked?
- Even if a cell phone is locked, you can still call 911. Cell phones will not allow the PIN code to be 911. 

What happens when I call 9-1-1 on my cell phone?
- You call is directed to the emergency authorities within your cell phone's area code (if you're using your carrier's service). It will then be directed to your local emergency service dispatch. Your cell phone will automatically enter the "emergency mode" and you will hear a BEEP and then you will not be able to access any other programs on your cell phone. You will only be able to call 911, unless you exit out of "emergency mode."

Does GPS automatically activate on my phone when I dial 911?
- When you make an emergency 911 call, the GPS feature on your phone will begin to calculate your approximate location (given that your cell phone has GPS). The emergency dispatch is also able to track your location based on the closest cell phone tower, but it's not always 100% correct.
 
Why can't I access my phone when it's in emergency mode?
- Cell phones are programmed to only allow access to 911 to conserve your cell phone's battery power. 

How do I get out of emergency mode on my cell phone?
- Some cell phones will prompt you to push a soft key. Some cell phones require you to hit *, # (star, pound). If your cell phone does not clear out of emergency mode, you can power off your phone. If you're not able to power off your phone, you made need to remove your battery.

Can I call 911 on an airplane with my cell phone?
- It depends how fast and high the airplane is flying. If the speed and height of the airplane is low, then you may be able to place your call. 
 

Mobile phones can be used in countries with different emergency numbers. A traveller visiting a foreign country does not have to know the local emergency numbers, however. The mobile phone and the SIM card have a preprogrammed list of emergency numbers. When the user tries to set up a call using an emergency number known by a GSM or 3G phone, the special emergency call setup takes place. The actual number is not even transmitted into network, but the network redirects the emergency call to the local emergency desk. Most GSM mobile phones can dial emergency calls even when the phone keyboard is locked, the phone is without a SIM card, or an emergency number is entered instead of the PIN.

Most GSM mobile phones have 112, 999 and 911 as pre-programmed emergency numbers that are always available. The SIM card issued by the operator can contain additional country-specific emergency numbers that can be used even when roaming abroad. The GSM network can also update the list of well-known emergency numbers when the phone registers to it.

Using an emergency number recognized by a GSM phone like 112 instead of another emergency number may be advantageous, since GSM phones and networks give special priority to emergency calls. A phone dialing an emergency service number not recognized by it may refuse to roam onto another network, leading to trouble if there is no access to the home network. Dialing a known emergency number like 112 forces the phone to try the call with any available network.

On some networks a GSM phone without a SIM card may be used to make emergency calls and most GSM phones accept a larger list of emergency numbers without SIM card, such as112, 911 118, 119, 000, 110, 08, and 999. However, some GSM networks will not accept emergency calls from phones without a SIM card, or even require a SIM card that has credit. For example, Latin American and UK GSM networks typically do not allow emergency calls without a SIM. Also, GSM phones sold in some countries like Singapore do not accept 112 as an emergency number if they have a SIM card inserted.

The GSM phones may regard some phone numbers with one or two digits as special service codes. It might be impossible to make an emergency call to numbers like 03 with a GSM phone.

In the United States, the FCC requires networks to route every mobile-phone and payphone 911 call to an emergency service call center, including phones that have never had service, or whose service has lapsed. As a result, there are programs that provide donated used mobile phones to victims of domestic violence and others especially likely to need emergency services.

Mobile phones generate additional problems for emergency operators, as many phones will allow emergency numbers to be dialed even while the keypad is locked. Since mobile phones are typically carried in pockets and small bags, the keys can easily be depressed accidentally, leading to unintended calls. A system has been developed in the UK which connects calls where the caller is silent to an automated system, leaving more operators free to handle genuine emergency calls