The feature described here applies is an useful functionality to debug in-call issues and simplify the work of searching events into the system logs.
In order to easily find into the logs when a particular issue happen you can press the pound key (#) on the handset, when you press such key a particular event is logged into the syslog:
In this case the user with SIP account 500 using the handset identified with Pmid 00001 pressed the Call Marker (#) during a call:
2017-11-16T13:43:28Z 172.16.18.12 000413610ff5 Umber 00512 -[ Call Marker user 500 Pmid:00001 CallId#0007] '
NOTE: this feature works only when you press the call marker during a call