A friend has asked about the best way to change all the 3 phones in her
small office so that they can be used with headsets, but also with some
sort of easy way for a young person on work experience to be able to
listen in to the conversations.

As far as I know, the phones are used one at a time and are all hanging
across the one line, so it just seems to be a matter of either buying
phones with one headset socket and perhaps hanging some sort of phone
listening device across the incoming line, or maybe finding phones with a
second headset socket.

It might be nice to have some sort of wireless monitoring for the snoop
person to save a lot of running around when the phone rings.

I see that Sanyo make a cheap recording interface for hanging across the
incoming line. Has anyone ever used one of these? What about the level
differences between incoming and outgoing speech and is it microphone or
line level? Are there likely to be any issues with REN numbers?

Alternatively, can anyone suggest any simple commercial system that might
achieve something like this?