Conference call outside phone number

[BrentNL]

I must be missing something, but after a lot of time researching I can't find the answer. How do you get outside callers to connect to the conference line? My generated emails have the conference ID number, but not an outside dial in number, such as (555) 555-1212. See below:

Hi,
You are invited to attend the following conference call: Test
To participate, please dial and enter this Conference ID ###387 to join the conference call at the specified time.
Internal employees can join the conference by dialing 7000 and entering conference ID ###387 when prompted.
Conference description: This is a test audio conference.

 

[Saqqara]

You have not mentioned which version

In 15.5, settings - conferencing - audio. Type in the external number routed to extension 700 (default)