You have to have a license that includes creating meetings in Skype for Business (or Teams when they move to Teams) to be able to add Audio Conferencing Add-On License. F1 does not include that, with a F1 license you are allowed to join meetings but not create them.
You also have the option to use Communication Credits with E1, E3 and E5. When this is assigned to a user they can use dial-in in their meetings and you will pay per minute for each caller that calls in to a meeting. This could be cheaper if you have users that only hosts a few meetings per month with few dial-in callers.
Best Response confirmed by
saikelu (Occasional Contributor)