Think of PSTN usage records as something which specify a class of call permitted in an organisation such as internal, local or international. Such records are used to associate with voice policies* and routes** to dictate usage of specific (gateway) routes by authorised users. A call can only be successful when a dialed number matches a route in a PSTN Usage.
A list of Public Switched Telephone Network, PSTN Usage records is accessible using the appropriately named Get-CsPstnUsage cmdlet. Because Global is the only identity applicable (to PSTN usages), you can basically retrieve available records via (Get-CsPstnUsage).Usage.
It is not possible to inspect the relationship between PSTN Usage, voice policy or route with a statement like Get-CsPstnUsage CH_Zurich. To resolve this, use Get-CsVoiceRoute or Get-CsVoicePolicy.
* assigned at the site or user scope where each voice policy must be linked to at least one PSTN usage record
** routes are assigned to telephone numbers
REFERENCE
– Configure voice policies, PSTN usage records, and voice routes in Skype for Business
– PSTN usage records in Lync Server 2013