There are a few things that could cause one extension to not ring when all the others are. Here is a list of the most frequent causes and their solutions.
Local Network Issues
The device or PC could be having local internet issues. Check to make sure it has an active internet connection.
Its possible the device dropped registration. To quickly re-register, you can power cycle the device.
Extension is Not in Ring Group/Queue
You can use this guide to check if its in the list.
Do Not Disturb
If the device has "do not disturb" enabled, this will prevent inbound calls from ringing the extension
There are two different "do not disturb" settings. One is directly on the phone and system, the other can be caused by *79 dialed directly onto the extension.
If none of these help resolve the issue, you can always reach out to our support team for assistance.