Also want to see call details like which extension is calling which number, call duration and which trunks are being used? See How to view active calls details on FreePBX instead.
In order to view how many calls are active concurrently on FreePBX, go to Admin -> Asterisk CLI
and enter
core show calls
Now click Send command on the right:
This will display, for example
1 active call 6 calls processed
indicating that one call is currently going on.
If no calls are currently in process, it will display
0 active calls 6 calls processed