One thing to note is that sometimes a 911 call might go through even though your phone shows no reception...

Inactive telephones

In the U.S., FCC rules require every telephone that can access the network to be able to dial 9-1-1, regardless of any reason that normal service may have been disconnected (including non-payment) (This only applies to states with a Do Not Disconnect policy in place. Those states must provide a "soft" dial tone service, details can be found at http://www.fcc.gov/Bureaus/Common_Carrier/Reports/FCC-State_Link/IAD/pntris99.pdf) On wired (land line) phones, this usually is accomplished by a "soft" dial tone, which sounds normal but will allow only emergency calls. Often, an unused and unpublished phone number will be issued to the line so that it will work properly. With regard to mobile phones, the rules require carriers to connect 9-1-1 calls from any mobile phone, regardless of whether that phone is currently active. The same rules for inactive telephones apply in Canada.

When a cellular phone is deactivated, the phone number is often recycled to a new user, or to a new phone for the same user. The deactivated cell phone will still complete a 911 call (if it has battery power) but the 911 operator will see a specialized number indicating the cell phone has been deactivated. It is usually represented with an area code of (911)-xxx-xxxx. If the call is disconnected, the 911 operator will not be able to connect to the original caller. Also because the cell phone is no longer activated, the 911 operator is often unable to get Phase II information.