Microsoft recently announced that they would be rolling out reports that show PSTN usage as well as users who are blocked from making PSTN calls.
If you are a global admin of a tenant that has PSTN-calling enabled, you will be able to view a PSTN usage report in the Microsoft Teams admin centre.
You will be able to view detailed calling activity for both Microsoft calling plans and for Direct Routing if you use your own telephony carrier.
Admins can also view the PSTN blocked users report to see which users are subject to outbound call controls.
To access these reports in the Microsoft Teams admin centre, select Analytics & reports in the left navigation. Under Report, choose the report you want to run.