Calls between Skype for Business users in your business are free, but if you want your users to be able to call regular phones outside of your business, you need to assign them a PSTN Calling plan. You can buy and assign different PSTN Calling plans to different users, depending on the needs of your organization. What is PSTN Calling? However, your organization has a certain amount of minutes for them, so if you run out of minutes, you will need to set up PSTN Consumption billing and assign a Skype for Business PSTN Consumption license to them.
More licensing information can be found here.
Want to know how much a PSTN Calling plan costs and how to buy it? See PSTN Calling plans for Skype for Business.
After you buy your PSTN Calling plans and assign the licenses, it's recommended that you set up PSTN Consumption billing and then assign that license to your users that will be using PSTN Calling or dialing-out from dial-in (PSTN) conferences.
Assign a Skype for Business PSTN Consumption license
Sign in to Office 365 with your work or school account.
In the left navigation of the Office 365 admin center, go to Users > Active users and then select the user or users from the list.
Tip: If you are assigning licenses to up to 20 users at the same time, you can use the Select a view drop-down then choose one of the options or create your own view. Then click Edit > Next > Next then select the license and click Submit.
In the Action pane under Product licenses, click Edit.
On the Product licenses page, toggle Skype for Business PSTN Consumption to On to assign this license.
Note: Even if you have users that are assigned an Enterprise E5 license it's still recommended that you do this.
After you assign these licenses, you will need to get your phone numbers for your organization, and then assign those numbers to the people in your organization.
For step-by-step instructions, see Set up PSTN Calling for Skype for Business.
Tips on assigning PSTN Calling plan licenses
IMPORTANT: For you to see the Voice option in the left navigation in the Skype for Business admin center, you must first buy at least one Enterprise E5 license, one Skype for Business Cloud PBX add-on license, or one Skype for Business PSTN Conferencing add-on license.
You will get an error message if you haven’t purchased the correct number of licenses. If you need to buy more licenses to enable a person for PSTN Calling, choose Buy more.
Because of the latency between Office 365 and Skype for Business Online, it can possibly take up to 24 hours for users to be enabled for PSTN Calling after you assign them licenses. If, after 24 hours a user isn't enabled for PSTN Calling, please call us.
Use Windows PowerShell to automate this task
For a sample script, see Assign Cloud PBX licenses and PSTN Calling plans to multiple users.