FAX Support - What span do I select when Running a PRI/BRI Debug?

Which span you choose depends on how many spans there are connected on the card and if the call is an inbound or outbound call. You can check under Setup>Telephony Cards and see how many spans are on a card. 

If it is an inbound call it starts on the last channel and uses each next available channel in descending order. If it is an outbound call it starts on the first channel and uses the next available channel in ascending order.

The call logs can be useful in seeing what channel is used by incoming or outgoing calls. Go to Reporting>Call Logs, select today's date and All Call, then View Log. You will see a list of calls starting with the most recent. By clicking on the notepad icon on the right of the log entry, you can see what channel an outgoing or incoming call used.