Some offices or businesses may require you to dial 9 to get an outside line to dial out. If you happen to dial 9 to get an outside line and 1 for a long distance number, the 1 could accidentally get pressed twice or be held down too long; therefore 911 is actually dialed.
Some business phone systems also require that you dial a 9 before you dial 911 for emergency assistance. You should check with someone responsible for maintaining your telephone system to see if dialing a 9 before 911 is a requirement for your office. If it is, make sure that these instructions are posted in plain view at each phone for employees to see in case they need to call 911.