We have about 15 users total.
Only about 12 of those call out and need voicemail.
What do we do about the other 3? Seems silly to pay for a seat if they are not calling out.
Each account is based on your individual order agreement, but I can tell you that with my company extensions that have no associated inbound/outbound calling and no meeting/conferencing capabilities are at no cost. You should speak to your Sales Account Manager to confirm.
If you have a RingCentral account, we charge by Digital Lines as opposed to "seats". What this means is that any user you add that does not have a phone and a phone number is not an additional charge.
The three users that don't need phones can be added as users without phones or a phone number. They will still be able to login, say, to administrate the account or to view call logs and reports. Make sure that you select "Add Users Without Phones" when making users like this.
We have more information on these subjects at our Digital Lines Overview and Adding User Extensions articles.
If (as I expect) you want these three users to have phones as well, you will be out of luck with RC. They don't provide true extensions, contrary to many other VoIP providers. I ended up putting a few of our deskphones with another provider that provides unlimited free extensions (and they still get inbound/outbound/voicemail). Saves me about $100/month.
If you haven't yet signed up make sure you have a plan for this before diving in. From experience, I can tell you that they won't offer you any alternative besides paying per month for each phone.