Reserved Extensions
| Extension | Reserved Purpose |
| 00 – 99 | Internal Use |
| 411 | Local Directory Assistance |
| 611 | Customer Service |
| 811 | Call Before You Dig |
| 911 | Emergency |
| 933 | Emergency Testing |
| 988 | National Suicide Hotline |
| 700 – 799 | Call Parking |
| 900 – 999 | Internal Use |
| 5000 / 9900 | Unregistered Login to Voicemail (prompted for acct # and password) |
| 5001 / 9901 | Registered Login to Voicemail (prompted for password only) |
| 5002 / 9902 | Hot Desking Sign In |
| 7000 – 7999 | Unusable due to 7+extension rule to transfer a call directly to voicemail |
| 9900 – 9999 | System Features |
Best Practice
| Extension | Usage |
| 100 – 599 | User Extensions |
| 700 – 719 | Call Parks – No Ringback |
| 720 – 799 | Call Parks – With Ringback |
| 800 – 899 | User Extensions |
| 1000 – 4999 | User Extensions |
| 5003 – 6999 | User Extensions |
| 8000 – 8999 | User Extensions |
| 9000 – 9899 | Special Extensions |
| – 9000 – 9099 | Auto Attendants |
| – 9100 – 9199 | Call Queues |
| – 9200 – 9299 | Call Routing |
| – 9300 – 9499 | Forwarding Extension |
| – 9500 – 9599 | Virtual Extension |
| – 9600 – 9899 | Future Use |