Hi Ggiannattasio,
Thank you for contacting Cisco Support Community.
As per the Cisco 3905 Data sheet:
http://www.cisco.com/en/US/prod/collateral/voicesw/ps6788/phones/ps7193/ps11542/data_sheet_c78-651588.pdf
++ Call manager express doesnt enforces phone licenses, however please ensure you do have paper license
++ If you are running IOS version 15.x, they enforce the UCK9 license. Without it installed, you wont be able to configure CME
++ To support Cisco 3905, you will need CME version 8.8 which is IOS version: 15.2.(1)T
Below is the reference link for 3905 supported cme version:
http://www.cisco.com/en/US/docs/voice_ip_comm/cucme/requirements/guide/cme88spc.html
Hope the above was informative.
Thanks and Regards
Prashanthi Velpula