Calls to other Skype for Business users are free, but if you want your users to be able to call regular phones, and you don't already have a PSTN service provider to make voice calls, you need to buy a Skype for Business PSTN Calling plan. What is PSTN Calling?
Here are the calling plans options:
Skype for Business PSTN Domestic Calling: Licensed users can call out to numbers located in the country/region where they are assigned in Office 365.
Skype for Business PSTN Domestic and International Calling: Licensed users can call out to numbers located in the country/region where their Office 365 license is assigned to the user in the Office 365 admin center > Assign License > Set user location, and to international numbers in 196 countries/regions.
Country/region amounts of monthly minutes for each organization and country/region are located here.
Notes: The country/region is based on the location of the user's license in the Office 365 admin center > Active Users and not the billing address listed under the Organization Profile in the Office 365 admin center.
How to buy a PSTN Calling plan
You must first purchase a Cloud PBX add-on license. To do that, sign into the Office 365 admin center and choose Billing > Subscriptions > Add-ons > Buy add-ons.
Important: Depending on your plan, you may need to buy more add-ons before you can buy Cloud PBX. To learn more, see Which add-on licenses do I need to buy to get more features?
When you buy your Cloud PBX add-on licenses, you'll see an option to add a PSTN Calling plan at the same time.
If you want to buy a PSTN Calling plan at a later time, at the Office 365 admin center, choose Billing > Subscriptions > Add-ons > Buy add-ons and you'll see the PSTN Calling plans there.
You can buy and assign different PSTN Calling plans to different users, depending on the needs of your organization. After you select the PSTN Calling plans you need, proceed to checkout. You assign the plans to each user in the Office 365 admin center. To learn how, see Assign Skype for Business licenses.
Do you have a PSTN service provider that provides on-premises PSTN connectivity for hybrid users?
If so, you don't need to buy a Skype for Business PSTN Calling plan. Office 365 Enterprise E5 includes Cloud PBX, so you can proceed to checkout.
Then, assign the Enterprise E5 or Cloud PBX add-on licenses to users in the Office 365 admin center. To learn how, see Assign Skype for Business licenses.
Prices for PSTN Calling plans: Click and scroll to the bottom of the page.
For more information
Here are more articles that explain how to set up your PSTN Calling service:
Plan for Skype for Business Cloud Connector Edition: This article explains how to set up Cloud PBX with your own PSTN connectivity.