Any organisation with at least one license including Phone System (e.g. Office 365 E5), or has Phone System as an add-on (e.g. to Office 365 E1/E3) are entitled to 25 x Phone System Virtual User Licenses at no cost.
For every ten additional Phone System licenses added, an organisation is entitled to one more Phone System Virtual User License.
From the Office 365 Admin Center, go to Billing > Purchase Services > Add-ons.
Note you need to be using Preview Mode.
Select Phone System – Virtual User.
Enter the number required, click Check out now.
Ensure “Automatically assign to all of your users with no licenses” is unchecked.
Note you need to have a valid payment method on the account, even though this is a free license.
This is unavailable using a CSP subscription. Update: Phone System – Virtual User licenses can now be provisioned via CSP.