as far as I know you dont need additional lics for that.
make sure you have restricted software installed, otherwise you will be flogging a dead horse.
This requires export controlled functionality in smart licensing.
To run CUCM 12.0 in Mixed Mode, it is necessary to successfully register the CUCM 12.0 with the "restricted" Registration Token from Smart Account. If you already have a Smart Account with Export functionality allowed, and you are running CUCM 12.0 in Mixed mode, make sure the token option checkbox for "Allow export-controlled functionality" is checked (which will generate a restricted Registration Token).
Hope this helps.
Cisco Unified Communications Manager, Release 10.5(2)SU6 requires an encryption license to run the product in mixed mode.Check Release Notes for Cisco Prime License Manager, Release 10.5(2)SU1a .
As per Cisco TAC 10.5 does not require Encryption License and tokenless. see link below for your reference send to me by Cisco TAC.
Your TAC engineer is wrong, you DO need an encryption license on 10.5(2)SU6, see slide 114