Hi Steffen,
You cannot add/delete phones in CCM 5.X until the temporary (upgrade) License file has been registered with Cisco via CCO. Once you have received a permanent License file via email and installed it you should be good to go. Have a look at the following info;
Migrating from Cisco CallManager 4.0(x) to 5.0(x)
When you migrate from Cisco CallManager version 4.0(x) to 5.0(x), the licenses that are required for existing phones and existing Cisco CallManager nodes is calculated and an intermediate file (XML file) that contains these license counts will be generated during the Cisco CallManager migration process. These licenses are given free of cost because you are already using these phones for Cisco CallManager version 4.x. If you are provisioning new phones and nodes after migrating to Cisco CallManager 5.0(x), you need to paste the intermediate license file in the License Registration window on CCO.
From this good doc;
http://www.cisco.com/en/US/products/sw/voicesw/ps556/products_administration_guide_chapter09186a0080531ebb.html#wp1100548
Hope this helps!
Rob