If you use IVR (Interactive voice response) on your Asterisk / Trixbox setup, Enable Direct Dial option allows callers to directly dial extensions.

By default it doesn’t allow to dial-in into conferences. If you try you will get response similar to:
“That option is invalid. Please try again“

This can be changed by adding following to your /etc/asterisk/extensions_custom.conf file:

[ivr-2-custom]
include => ext-meetme

ivr-2  can be different in your case. You can find what it is in /etc/asterisk/extensions_additional.conf

Trixbox 2.6
Asterisk 1.4