# System Service Extension Numbers

### Reserved System Extension Numbers

PortSIP PBX assigns **default extension numbers** to built-in system services.\
These extension numbers are **reserved** and **cannot be used** when creating user extensions.

#### Default System Extensions

* **Call Announcement Service**: `555`\
  To change this number, go to **Advanced Services > Automatic Callback**.
* **Call Park Service**: `666`\
  To change this number, go to **Advanced Services > Call Park > Global Settings**.
* **Voicemail Service**: `777`\
  To change this number, go to **Advanced > Voicemail**.
* **Call Monitor Service**: `888`\
  To change this number, go to **Contact Center > Monitor Service**.