911 is the number to dial for emergency services in the USA and Canada.
911 calls are answered at a PSAP.

911 History and Facts

See Also
National Emergency Number Association (NENA)
911 Emergency Alert Systems: Emergency Notification Systems from Database Systems Corp.
PSAP
VOIP 911 Service Providers
Decision on VOIP 911 service by CRTC in Canada.
News release/Summary of CRTC Decision on 9-1-1 Emergency Services for
VoIP Service Providers
US FCC Requires Interconnected VoIP Providers to Provide Enhanced 911 Service. News release (PDF)

E911 Support on Asterisk Open Source PBX - E911 support via ISDN PRI
Wikipedia 9-1-1