Do you need to look at call logs ? If no, then you can delete them
Do you have a company requirement to look at calls going back past a certain date ? - If no, then you can safety deleted them
As above, the more info in the call logs - the longer the backups takes and the reverse , restores will take the same time
The button on the dashboard will clear all call logs, to delete a certain period you do this via Call Reports -> Settings tab