A lot depends on what settings are on the switch that is at the other end of your network connection. It is very common for your network admin to only allow a single phone. It is alos generally necessary to register the phones with the Cisco UC Manager to add and associate the number and features that are necessary.
There may also be other settings like VLAN etc. that are not compatible between the switch managed by your IT department and the one you installed. You need to check with your IT department to determine the issue.