What you trying to do is called "call monitoring" then conferrence the different calls that are being monitored. You can actually monitor the calls in another conference bridge if you like or just simply call monitor each one from you phone or a conference phone by pressing the hock/flash to conference them all.
Cisco have notes on it at the link below.
http://www.cisco.com/en/US/products/ps6706/prod_technical_reference09186a008086110b.htmlGood luck!