---
title: "Reserved Extensions"
slug: "reserved-extensions"
updated: 2025-07-23T20:46:34Z
published: 2025-07-23T20:46:34Z
canonical: "docs.ucstack.io/reserved-extensions"
---

> ## Documentation Index
> Fetch the complete documentation index at: https://docs.ucstack.io/llms.txt
> Use this file to discover all available pages before exploring further.

# Reserved Extensions

The following extensions are **not allowed** to be used by any **user**.

| Extension (range) | Reason |
| --- | --- |
| 0-99 | No extensions less than 3 digits |
| 311 | Local Community Hotline (where supported) |
| 411 | Local Directory Services (where supported) |
| 511 | Road Conditions (where supported) |
| 611 | Support Contact |
| 711 | TTY Relay Services (where supported) |
| 811 | Health Services (where supported) |
| 911 | Emergency Services |
| 988 | Mental Health and Suicide Hotline |
| 80000-89999 | Reserved for future use |
| 90000-99999 | System Features / Reserved Extensions |

Additionally, Any number that starts with 9, and only contains 9's and 1's. For example:

> [!CAUTION]
> 9911
> 
> 991
> 
> 9111