The 7937 is like most other desk phones were it can only process one audio stream at a time. So if you want to conference two calls together, they need to be mixed at a central point. Generally it's best to use a conference bridge device like DSPs on a Cisco IOS router. You can however use the CallManager servers themselves to do this mixing (all calls must be G.711). When you look under the Conference Bridge section, you should see your CallManagers listed as a Software Conference Bridge. You need to make sure there is a Media Resource Group that has them added. Then add that Media Resource Group to a Media Resource Group List. Finally, add that Media Resource Group List to your Device Pool or the device itself. Now when you do a conference, the CallManager will be able to reference the bridges and utilize them to join the calls together.