Office 365 has many different subscription options, sometimes referred to as service plans or SKU’s (Stock Keeping Unit, a retail term for a specific unique product code).
These are bundles of services targeted at different business sizes, use cases and sometimes specific verticals like Education or Government.
Here is a complete list on docs.microsoft.com. You can also buy some standalone services, but it seems Microsoft is pushing more towards bundles.
In addition you can buy “Add-ons” to add some abilities not included in your bundle.
These Add-ons are only available to Enterprise plans:
US Government tenants (GCC, GCC High and DoD) can also get these features/add-ons, documentation here. Government organizations outside the US use the standard commercial cloud enterprise plans, but often in GoLocals data centres.
Audio Conferencing is available for $4 per user per month or on a pay per minute basis
Audio Conferencing and Phone System add-ons are not available for Business Plans
What is confusing, is that Business Essentials and Business Premium come with Microsoft Teams (and previous to this Skype for Business), which at a base level includes IP/computer-based VoIP calling and conferencing, but Microsoft won’t allow buying the add-on’s that allow you to add PSTN dial-in numbers to your Teams conference (Microsoft Audio Conferencing), or make your Teams client into a real phone (Phone System) or Microsoft to be your calling plan with minutes and a phone number (calling plan).
This means if you are a business of say 50 users, you can buy Office 365 business premium for $12.50 a user, but if you also need PSTN dial-in numbers for your conferences you can’t add the $4 or pay per minute add-on. You would have to move to Office 365 E3 (to get the equivalent Office apps) at $20, then the $4 or (pay per minute) as well, so an extra $11.50 per user per month. E3 is great if you already want those features, but in this case, the user just wants PSTN dial-in numbers for their conferences.
Same story if you want phone system ($7.50 to get to E3, then $8 for phone system and $12 for your calling plan).
Why are these only available to enterprise plans?
The honest answer is I don’t know. I’ve asked but can’t get a clear answer. It might just be that Microsoft wants to keep those options to higher plans to sell at a higher price (fair enough, their choice), but it leaves the SMB space wide open for people to sign up for whatever conference provider they choose and various very competitively priced cloud phone systems.
Seems like a missed opportunity to make themselves the obvious choice for Business plan subscribers.
There is a public user voice asking for this, but at time of writing, it only has 1 vote.