Adding H323 extensions
From TD-er's Wiki
A H323 extension can be added in FreePBX as a custom extension. For example:
Extension number: 601 Displayname: Anneke h323 Dial: OOH323firstname.lastname@example.org (ip-address of machine where extension can be found)
Asterisk saves these extensions in the file extensions_additional.conf in the [ext-local] context.