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.
https://www.microsoft.com/en-us/microsoft-365/compare-all-microsoft-365-plans
Creating meetings is included in E1 (and E3, E5) so with this license you can buy the Add-On for Audio Conferencing.
https://www.microsoft.com/en-us/microsoft-365/business/office-365-enterprise-e1-business-software
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.
