Often times I have needed to check the Voice Routing to determine that the call is matched to the correct policies and routes and subsequently the correct gateway.
Looking into a simple PSTN call in OCSLogger simply search for 101 Progress Report
The Source is the FQDN of my Front End Pool
Phone Usage, Route and gateway are shown in this message as well as appName which tells that the call is OutBound.
You will get this SIP 101 Progress Report message for all Inbound, Outbound aswell as peer to peer calls