Guys, is it at all possible to have more than one extension to be able to pick up CO 1 at the same time ? I have a customer (in a house) that wants to be able to jump on the line from any phone so more than one person can listen in/talk. She seems to think the system can do it as she has one back in her other house in the US. (I personally think she has another type Panasonic system back home but anyway...)

Yes, as long as all stations have executive override enabled. Otherwise, you can press and hold down the CO line button in use and others can then join in by pressing the same button.