PSTN calling is an add-on telephone service that, when combined with Skype for Business Cloud PBX, can become your phone system. This is currently available for users in the United Kingdom, United States, Puerto Rico, France and Spain.
Microsoft now introduced this feature in Preview for The Netherlands and Ireland. To nominate your organization, make sure you’re prepared to deploy PSTN Calling to at least 50 users and are available to give feedback on the experience.
Based on what we’ve seen with France and Spain I expect general availability in two or three months. More information on https://www.skypepreview.com/