These options specify how call forwarding and simultaneous ringing of phones are routed: Route using the call PSTN usages: ...

These options specify how call forwarding and simultaneous ringing of phones are routed:
Route using the call PSTN usages: Use the same PSTN usages as typical voice calls.
Route to internal Lync users only: Route to just Lync users.
Route using custom PSTN usages: Use the specified PSTN usages for routing.
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