There is a device called echo. It's kinda expensive if I remember, and it is an add on to every phone, goes in place of an HHEU.
If they have analog lines, there is a device that we installed in a investment place, that will automatically pickup and record all incoming and outgoing calls. I do not remember what they were called.
Last option, if you have $50,000 to drop, is a server that half taps your digital phones. When a conversation is started it records and saves the conversations on the server. Any manager with access to the machine can review and playback any conversation from any extension at any time. If it is something you would like to invest in, I can get the name of the unit for you.